[0001] This description relates generally to a reconfigurable filter to enable an isolated amplifier to operate in any of multiple modes including a fully differential mode, a single-ended mode in which a single-ended output is generated from a differential input, as well as unipolar modes, to support multiple different products, including direct interface with a single-input analog-to-digital converter (ADC).

BACKGROUND

[0002] An isolated amplifier has an output separated from the amplifier’s input circuitry by an isolation barrier that can withstand high voltage. The isolation barrier separates parts of the system that operate on different common-mode voltage levels and protects the low voltage side from hazardous voltages and damage. The input of an isolated amplifier may be optimized for direct connection to a low-impedance shunt resistor or other low impedance voltage source with low signal levels. Applications such as isolated voltage or current sensing in various environments including automotive require excellent DC accuracy and low temperature drift.

[0003] Many isolated amplifiers used in such sensing applications provide a differential analog output between two output pins, which can interface with a differential input ADC. To interface with a single-ended-input ADC, however, such amplifier requires external circuitry to convert the amplifier output to a ground-referenced, singled-ended signal for input to the single-ended-input ADC. This external circuitry, however, requires additional space, increases complexity and adds cost to the overall product.

[0004] Another prior approach to interfacing an isolated amplifier with a single-ended-input ADC is to use to two such ADCs, one coupled to one of the amplifier’s differential outputs and the other coupled to the other of the amplifier’s differential outputs. This approach presents accuracy issues as the signal at each output pin of the amplifier measured against ground is not an accurate signal. Overhead is added in terms of space, complexity and cost to accommodate two ADCs and to address the accuracy issue.

[0005] In this context, features and aspects of the present description arise. SUMMARY

[0006] In an example, a reconfigurable filter comprises first and second inputs adapted to receive an input signal; a fully differential amplifier (FDA); and first and second reconfigurable resistancecapacitance (RC) networks. The FDA has an inverting input, a non-inverting input, an inverting output, and a non-inverting output. The inverting input is coupled to the first input, and the noninverting input is coupled to the second input. The first reconfigurable RC network is coupled to the non-inverting output, and the second reconfigurable RC network is selectively couplable to the inverting output. The reconfigurable filter is configurable to enable operation in any of multiple modes including a single-ended mode of operation and a differential mode of operation.

[0007] In an example, a system (e.g., which may be or include an isolated amplifier) comprises a digital-to-analog converter (DAC); a multiple feedback (MFB) filter; and a plurality of terminals including an output terminal, a multi-functional terminal, a voltage supply terminal, and a ground terminal. The DAC has a digital input, a DAC reference voltage input, a first output, and a second output. First and second inputs of the MFB filter are coupled to the first and second outputs of the DAC, respectively. The MFB filter includes a fully differential amplifier (FDA) having an inverting input, a non-inverting input, an inverting output, and a non-inverting output. The inverting input is coupled to the first input, and the non-inverting input is coupled to the second input. The MFB filter is configurable to enable an amplifier of the system to operate in any of multiple modes including a single-ended mode of operation and a differential mode of operation.

[0008] In an example, a method comprises controlling a first reconfigurable resistancecapacitance (RC) network of a filter of an amplifier to set the amplifier in a select mode of operation among multiple modes of operation; controlling a second reconfigurable RC network of the filter to set the amplifier in the select mode of operation; selecting a source of a reference voltage that is input to a digital-to-analog converter (DAC) of the amplifier based on the select mode of operation; and generating, by the amplifier, an output signal according to the select mode of operation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIG. l is a diagram of an example isolated amplifier.

[0010] FIG. 2 is a diagram of an example filter system that includes an example multi-way reconfigurable multiple feedback (MFB) filter to enable an isolated amplifier to operate in any of multiple modes.

[0011] FIG. 3 is a diagram of another example fdter system that includes an example multi-way reconfigurable MFB filter to enable an isolated amplifier to operate in any of multiple modes.

[0012] FIG. 4A is a diagram of an example isolated amplifier system including circuitry to enable direct coupling to a single-ended-input ADC.

[0013] FIG. 4B is a diagram of another example isolated amplifier system including circuitry to enable direct coupling to a single-ended-input ADC.

[0014] FIG. 5 is a flow diagram of an example method of setting an example multi-way reconfigurable MFB filter, such as that shown in FIG. 2 or 3, in a particular configuration to enable an associated isolated amplifier to operate in a specific mode.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0015] Specific examples are described below in detail with reference to the accompanying figures. In the drawings, corresponding numerals and symbols generally refer to corresponding parts unless otherwise indicated. The objects depicted in the drawings are not necessarily drawn to scale.

[0016] In example arrangements, systems, circuits and methods provide an isolated amplifier with a multi-way reconfigurable MFB filter to enable the isolated amplifier to operate in any of multiple modes including a fully differential mode, a single-ended mode in which a single-ended ground-referenced output signal is generated from an input signal, which may be a differential input signal, as well as various unipolar modes. Circuitry is also provided to enable direct coupling of the single-ended output to a single-ended-input ADC and to scale the single-ended, ground- referenced output signal with a reference voltage that matches the full-scale range of the ADC. The reference voltage may be provided through a dedicated or shared terminal (or pin). The arrangements advantageously support multiple different products and applications.

[0017] FIG. I is a diagram of an example isolated amplifier 100, which includes input-side circuitry 102 for processing a differential signal input at terminals INP and INN of isolated amplifier 100. Its terminal VDD1 is adapted to be coupled to a high-side voltage supply, which may be 3.3 V or 5 V, referred to GND1. Terminal GND1 of isolated amplifier 100 is adapted to be coupled to high-side ground. Isolated amplifier 100 further includes output-side circuitry 104 that is separated from input-side circuitry 102 by an isolation barrier 106. On the output side, terminal VDD2 is adapted to be coupled to a low-side voltage supply, which may be, for example, 3.3 V or 5 V. Terminal GND2 is adapted to be coupled to low-side ground. Tn isolated amplifier 100, terminals OUT(P) and OUTN/VREFIN serve multiple purposes. When isolated amplifier 100 is configured in differential mode, a differential output signal (OUTP - OUTN) is outputted at these terminals. When isolated amplifier 100 is configured in single-ended mode or in any of various unipolar modes, a single-ended output signal (OUT) is outputted at terminal OUT(P) referenced to GND2, while terminal OUTN/VREFIN functions as an input for an externally- supplied reference voltage (VREFIN) or midpoint reference. Thus, the OUTN/VREFIN terminal may be considered a multi-functional terminal. A multiple feedback (MFB) filter is provided as part of output circuitry 104 to configure isolated amplifier 100 in its various modes and facilitate low-pass filtering following D/A conversion.

[0018] FIG. 2 is a diagram of an example of such a multi-way reconfigurable MFB filter (or filter component) 204 within a filter system 200. Filter system 200 also comprises a digital-to- analog converter (DAC) 202 that drives filter component 204. DAC 202 has a digital input 206 at which a bitstream (bs) is input, and a DAC reference voltage input 208 at which a DAC reference voltage (VREF_DAC) is input based on a voltage signal that may be sourced differently depending on the mode of operation. The voltage signal is input to DAC 202 through a buffer 210. DAC 202 is configured to output a differential analog signal at first and second outputs 212 and 214, respectively. The bitstream (bs) may be the output of an analog-to-digital converter (ADC) (not shown) at the input side of isolated amplifier 100.

[0019] The first and second outputs 212 and 214 of DAC 202 are coupled to respective first and second inputs 216 and 218 of filter component 204. First output 212 is coupled to first input 216 via a first pair of resistors 222, and second output 214 is coupled to second input 218 via a second pair of resistors 224. Each of the resistors of the first and second pairs may have a resistance which is multiple of R, where R denotes a unit value of resistance, or other suitable resistance consistent with the teachings herein. A capacitor 226 may be coupled between a first node between the resistors of the first pair and a second node between the resistors of the second pair. In another example, capacitor 226 may be omitted. Another capacitor 228 is coupled between first and second inputs 216 and 218 of filter component 204. Capacitor 226, when used, may have a capacitance of nC, where n is a factor and C denotes a unit value of capacitance. Capacitor 228 may have a capacitance of «?C, where m is another factor that may be different than factor n. In some examples, the resistor in each of the first and second pairs that is coupled directly to DAC 202, as well as capacitor 226, may be separate from filter component 204.

[0020] Filter component 204 comprises a fully differential amplifier (FDA) 232 that can be reconfigured to drive OUTP only as an operational amplifier (op amp), a first reconfigurable resistance-capacitance (RC) network 234, and a second reconfigurable RC network 236. FDA 232 has an inverting input (-) and a non-inverting input (+), as well as an inverting output (-) and a non-inverting output (+). FDA 232 may be configured, via a switching mechanism, such that its inverting output can be deactivated (e.g., set to a high impedance state). This is exemplified by an external switch 238 coupled to the inverting output of FDA 232, although other techniques may be used to achieve deactivation. For example, one or more switches within FDA 232 may be employed to deactivate (float, tristate) OUTN without being series with that output. RC network 234 is coupled between the non-inverting output and the inverting input of FDA 232. In the external switch example, RC network 236 is selectively couplable to inverting output of FDA 232 via switch 238 and is also coupled to the non-inverting input of FDA 232.

[0021] Filter component 204 further comprises first and second integrator resistors 242 and 244, respectively. First integrator resistor 242 is coupled between first input 216 of filter component 204 and the inverting input of FDA 232, and second integrator resistor 244 is coupled between second input 218 of filter component 204 and the non-inverting input of FDA 232. Being coupled to the inputs of FDA 232, integrating resistors 242 and 244 may also be referred to as input resistors.

[0022] First RC network 234 includes a first variable resistive element 252 and a first variable capacitive element 254. First variable resistive element 252 is coupled between the non-inverting output of FDA 232 and input 216 of filter component 204, while first variable capacitive element 254 is coupled between the non-inverting output of FDA 232 and its inverting input. First variable resistive element 252 represents one or more resistors that may be configured to provide a specific resistance between the non-inverting output of FDA 232 and input 216 of filter component 204. Similarly, first variable capacitive element 254 represents one or more capacitors that may be configured to provide a specific capacitance between the non-inverting output and the inverting input of FDA 232.

[0023] Second RC network 236 includes a second variable resistive element 256 and a second variable capacitive element 258 having input ends coupled to input 218 of filter component 204 and to the non-inverting input of FDA 232, respectively. In an example, the output ends of second variable resistive element 256 and second variable capacitive element 258 are selectively couplable to the inverting output of FDA 232 via switch 238. In another example, as described above, the inverting output of FDA 232 may be activated (e.g., by an internal mechanism) to thereby activate second RC network 236. As is the case with first variable resistive and capacitive elements 252 and 254, second variable resistive and capacitive elements 256 and 258, respectively, represents one or more components. Accordingly, one or more resistors may be used to configure second variable resistive element 256 to provide a specific resistance in its electrical path, and one or more capacitors may be used to configure second variable capacitive element 258 to provide a specific capacitance in its electrical path.

[0024] Filter component 204 also includes a variable resistor 262, which may be one or more resistors, coupled between input 218 and a ground terminal (GND2). Terminal OUT(P) of filter component 204 is coupled to the non-inverting output of FDA 232. Terminal OUTN/VREFIN functions as an output terminal (OUTN) when filter component 204 is configured in differential mode, in which case switch 238 is closed (in the external switch example), coupling such terminal to the inverting output of FDA 232. In single-ended mode, switch 238 (when used) is open, in which case OUTN/VREFIN functions as an input terminal (VREFIN).

[0025] In addition to switch 238 (or other mechanism for activating/deactivating the inverting output of FDA 232), first and second RC networks 234 and 236 may also be configured. An example structure and control scheme for RC networks 234 and 236 is shown with respect to reconfigurable MFB filter (or filter component) 304 in FIG. 3, which is shown in the context of an example filter system 300.

[0026] In addition to showing example structure and operation of variable resistive elements 252 and 256, as well as variable capacitive elements 254 and 258, FIG. 3 shows additional components of filter component 304 that are also used in configuring isolated amplifier 100. Components of filter component 304 that are the same as, or substantially similar to, those of filter component 204 are identified with the same reference numerals as their respective counterpart components of filter component 204.

[0027] In example filter component 304, variable resistive element 252 includes resistor 302 that is selectively couplable between input 216 of filter component 204 and the output terminal (OUT(P) via a switch 305, which may be in the form of an n-channel metal-oxide-semiconductor (NMOS) field-effect transistor (NMOS transistor). When switch 305 is ON (conducting), resistor 302 is coupled between input 216 and OUT(P); otherwise the path of resistor 302 and switch 305 is open. In the example of FIG. 3, resistive element 252 further includes a pair of resistors 306 and 308, which may be configured for series operation or for use of only resistor 308 with resistor 306 shorted. Accordingly, (NMOS transistor) switch 310 is coupled across the terminals of resistor 306 to create a short circuit path when switch 310 is ON (conducting). Thus, by using switches 305 and 310, various different resistance values may be set for reconfigurable RC network 234, of which variable resistive element 252 is a part. In an example, the resistance of resistor 302 is 3R, and the resistance of each of resistors 306 and 308 is 1.5R with a series equivalent of 2 * 1.5R when switch 310 is OFF.

[0028] Reconfigurable RC network 234 also includes variable capacitive element 254, which, in example filter component 304, includes capacitors 312 and 314, as well as (NMOS transistor) switch 316. Depending on whether switch 316 is ON or OFF, the capacitance between the OUT(P) terminal (also the non-inverting output terminal of FDA 232) and the inverting input terminal of FDA 232 may be the total capacitance of capacitors 312 and 314 coupled in parallel (2C), or may be the capacitance of capacitor 314 (C).

[0029] Reconfigurable RC network 236 is similarly constructed, using switch control of multiple resistors and capacitors. In the illustrated example, variable resistive element 256 includes two resistors 322 and 324, as well as (NMOS transistor) switch 326. As configured, switch 326 may be used to set the resistance of reconfigurable RC network 236 to the sum of the resistances of resistors 322 and 324 (2 x 1.5R), or simply the resistance of resistor 324 (1.5R). In example filter component 304, variable capacitive element 258 of reconfigurable RC network 236 includes two capacitors 332 and 334, as well as (NMOS transistor) switch 336, which may be used to set the capacitance of reconfigurable RC network 236 to one of two different capacitances, i.e., C or 2C. [0030] In filter component 304, variable resistor 262 is implemented by a resistor and a switch coupled in series. In the illustrated example, variable resistor may have a value of 3R or essentially infinite (oo) depending on whether the switch is closed or open.

[0031] Example filter component 304 further includes circuitry for switching the source of VREF_DAC that is input to DAC 202. Such switching circuitry includes a resistor 342 coupled between terminal OUTN/VREFIN and a first input of a two-input multiplexer 344. The switching circuitry further includes a resistor 346 coupled in parallel with a capacitor 348 between the first input of multiplexer 344 and ground (GND2). A voltage source (VREF) 352 is coupled to the second input of multiplexer 344. The output of multiplexer 344 is coupled to the input of buffer 210, the output of which is coupled to the DAC reference voltage input 208. Multiplexer 344 selects the source of VREF DAC, depending on the mode of operation.

[0032] In an example, to configure amplifier 100 in differential mode to generate a differential output signal in response to a differential input signal, switch 238 is closed and reconfigurable RC networks 234 and 236 are configured as follows. In reconfigurable RC network 234, switch 310 is ON, switch 305 is ON, and switch 316 is ON, providing a resistance of 1.5R and a capacitance of 2C. In reconfigurable RC network 236, switch 326 is ON and switch 336 is ON, providing a resistance of 1.5R and a capacitance of 2C. In differential mode, the switch of variable resistor 262 is OFF, providing a resistance of infinity. Also, in differential mode, multiplexer 344 selects the input from voltage source 352 as the source of VREF_DAC.

[0033] The table below shows example configurations for the differential input mode, as well as other modes, each of which may be obtained by controlling the switches of filter component 304. In all the modes except differential, switch 238 is open. VREF_DAC is sourced through the VREFIN terminal of amplifier 100 for ratiometric modes. In single-ended (SE) mode, with ratiometric scaling (ratio), and in the unipolar ratiometric mode, the voltage signal input to buffer 210 is VREFIN/3, whereas in the other modes (fixed gain) the voltage signal input to buffer 210 is VREF. In either the unipolar 1.5V/V mode or unipolar 1V/V mode, the output swing may be offset by VREFIN or fraction thereof. The voltage Vciip is the absolute clipping voltage of the high-side (INN - INP), and VREF is an internal reference voltage.

[0034] FIG. 3 shows an example of how reconfigurable RC networks 234 and 236 may be structured. Other arrangements are possible consistent with the teachings herein. For example, a second-order MFB filter may be used, in which case capacitor 226 may be omitted. Each RC network may include any suitable combination of parallel and/or series coupled resistors, as well as any suitable combination of parallel and/or series coupled capacitors, along with controlling switches. Similarly, FIG. 3 shows an example configuration of variable resistor 262; however, it may be configured in any suitable way to obtain the desired resistance for a given mode of operation.

[0035] FIGS. 4A and 4B show different examples of larger systems (i.e., environments) 400 and 450, respectively, in which an isolated amplifier 100 is configured in single-ended mode to generate, in response to a differential input signal, a single-ended, ground-referenced output signal at its output (OUT) that is coupled directly to a single-ended-input ADC 402, which may be part of a microcontroller unit (MCU). In both systems 400 and 450, the multi-functional terminal of isolated amplifier 100 (OUTN/VREFIN) is configured as a reference input terminal (VREFIN) to receive a reference voltage (VREF).

[0036] Both systems 400 and 450 include input circuitry 404. In the examples of FIGS. 4A and 4B, input circuitry 404 includes a low-impedance shunt resistor (RSHUNT) in a current path to generate an input signal delivered to input terminals IMP and INN of isolated amplifier 100. One end of RSHUNT is coupled to the input-side ground terminal (GND1) of isolated amplifier 100. Input circuitry 404 may be different than that shown, depending on application. In the illustrated example, the differential input signal is approximately 640 mV peak-to-peak (pp). [0037] In both systems 400 and 450, the input-side voltage supply terminal (VDD1) of isolated amplifier 100 may be coupled to a voltage supply (not shown). Also, in both systems 400 and 450, the output-side voltage supply terminal (VDD2) of isolated amplifier 100 is coupled to the voltage supply terminal (VDDA) of the MCU, and both such terminals are commonly coupled to a voltage supply, which may be within a range of approximately 2.7 V to 5.5 V, e.g., 3.3 V, and which may be different than the voltage supply of the input-side. The output-side ground terminal (GND2) and a ground terminal of the MCU are both coupled to a common ground. The voltages indicated in FIGS. 4A and 4B are examples.

[0038] In system 400 of FIG. 4A, the VREFIN terminal is coupled to both a VREFHI terminal and the VDDA terminal of the MCU. In contrast, in system 450 of FIG. 4B, the VREFIN terminal is only coupled to the VREFHI terminal, and a voltage source 452 is coupled between the VREFIN terminal and the GND2 terminal. Voltage source 452 is configured to generate an external reference voltage (VREF_EXT) that is supplied to isolated amplifier 100 via its VREFIN terminal and to the MCU via its VREFHI terminal. In an example, VREF EXT may range between approximately 2.7 V and the voltage of VDD2; other ranges are possible. In an example, VREF_EXT is less than the voltage supplied to VDD2 and VDDA.

[0039] The single-ended, ground-referenced output signal generated by each of systems 400 and 450 is scaled with the differential input signal (bipolar inputs). In the examples of FIGS. 4A and 4B, the output voltage of isolated amplifier 100 (V(OUT, GND2)) is substantially centered about VREFIN/2. At zero input, amplifier 100 outputs approximately VREFIN/2; in response to positive input signals, the output of amplifier 100 ranges between approximately VREFIN/2 and VREFIN; and in response to negative input signals, the output of amplifier 100 ranges between approximately VREFIN/2 and ground (zero). Moreover, in example systems 400 and 450, V(OUT, GND2) = (Vin*A +1)*(VREFIN - GND2)/2, where Vin is VINP - VINN and A represents a fixed gain value; in an example, A = 1/Vciip. Other input/output signal combinations are possible. For example, in the case of unipolar fixed gain modes, the output voltage is driven as follows. At zero input, amplifier 100 outputs a value that closely follows VREFIN or fraction thereof, and from positive input signals, amplifier 100 outputs between zero and A*VREF, where A may be 1.5 or 3 in the examples in the table above.

[0040] FIG. 5 is a flow diagram of an example method 500 of operating an isolated amplifier (e.g., isolated amplifier 100) with a reconfigurable MFB filter (e.g., MFB filter 204 and/or 304). [0041] In operation 502, a first reconfigurable resistance-capacitance (RC) network of a multiple feedback filter of an amplifier, e.g., an isolated amplifier, is controlled to set the amplifier in a select mode of operation among multiple modes of operation. In operation 504, a second reconfigurable RC network of the multiple feedback filter is controlled to set the amplifier in the select mode of operation. The controlling or configuring of the first and second reconfigurable RC networks may be carried via switches, e.g., NMOS transistors, as described above. In operation 506, a source of a reference voltage that is input into a digital-to-analog converter (DAC) of the amplifier is selected based on the select mode of operation. With the amplifier so configured, in operation 508, an output signal is generated according to the select mode of operation.

[0042] FIG. 5 depicts one possible set and order of operations. Not all operations need necessarily be performed in the order described. Some operations may be combined into a single operation. Additional operations and/or alternative operations may be performed.

[0043] Examples of circuitry, systems and methods provide a multi-way configurable isolated amplifier that supports unipolar and bipolar sensing applications. By employing a multi-way reconfigurable MFB filter, the amplifier is able to utilize the full dynamic range of the downstream ADC and therefore maximizes measurement resolution. No external circuitry is required to generate a single-ended output signal, thereby reducing board space, system cost. Advantageously, a single piece of silicon can be configured to operate in any of the modes described above. Measurement accuracy is also improved. Because of the ratiometric scaling at the output in single-ended mode and unipolar ratio mode, it is possible to suppress or cancel errors between the reference voltage of the host system and of the DAC reference voltage of the isolated amplifier. Gain and gain drift errors of the signal chain are defined only by the input-side circuits of the isolated amplifier.

[0044] The term “couple” is used throughout the specification. The term and derivatives thereof may cover connections, communications, or signal paths that enable a functional relationship consistent with this description. For example, if device A provides a signal to control device B to perform an action, in a first example device A is coupled to device B, or in a second example device A is coupled to device B through intervening component C if intervening component C does not substantially alter the functional relationship between device A and device B such that device B is controlled by device A via the control signal provided by device A. [0045] A device that is “configured to” perform a task or function may be configured (i.e. programmed and/or hardwired) at a time of manufacturing by a manufacturer to perform the function and/or may be configurable (or re-configurable) by a user after manufacturing to perform the function and/or other additional or alternative functions. The configuring may be through firmware and/or software programming of the device, through a construction and/or layout of hardware components and interconnections of the device, volatile or non-volatile memory elements, or a combination thereof.

[0046] As used herein, the terms “terminal” and “pin” also encompass “node”, “interconnection” and/or “lead”. Unless specifically stated to the contrary, these terms generally mean an interconnection between or a terminus of a device element, a circuit element, an integrated circuit, a device or other electronic or semiconductor component.

[0047] A circuit or device that is described herein as including certain components may instead be adapted to be coupled to those components to form the described circuitry or device. For example, a structure described as including one or more semiconductor elements (such as transistors), one or more passive elements (such as resistors, capacitors, etc.), and/or one or more sources (such as voltage and/or current sources) may instead include only the semiconductor elements within a single physical device (i.e. a semiconductor die and/or integrated circuit (IC) package) and may be adapted to be coupled to at least some of the passive elements and/or the sources to form the described structure either at a time of manufacture or after a time of manufacture, for example, by an end-user and/or a third-party.

[0048] While the use of n-channel MOSFETs is described herein, other types of transistors (or equivalent devices) may be used instead or in combination. Circuits described herein are reconfigurable to include the replaced components to provide functionality at least partially similar to functionality available prior to the component replacement.

[0049] Uses of the phrase “ground” in the foregoing description includes a chassis ground, an Earth ground, a floating ground, a virtual ground, a digital ground, a common ground, and/or any other form of ground connection applicable to, or suitable for, the teachings of this description. Unless otherwise stated, “about,” “approximately,” or “substantially” preceding a value means +/- 10 percent of the stated value.

[0050] Modifications of the described examples are possible, as are other examples, within the scope of the claims.