MULTI-LEVEL SIGMA-DELTA ADC WITH REDUCED QUANTIZATION

LEVELS

Technical Field

The present invention relates to Analog-to-Digital

Converter circuits (ADCs) and particularly to a multi ^¬ level sigma-delta ADC with reduced quantization levels.

Background art

Analog-to-Digital Converter circuits (ADCs) are often implemented with Sigma-Delta topologies when high accuracy is required.

An example of application is in the audio field, where Sigma-Delta ADCs are the most commonly used.

As known, this topology of converters transforms an analog input signal to a digital stream of words with a low number of bits and a spectrally-shaped quantization noise .

The first Sigma-Delta converters had a single bit output (2 levels) , then they evolved to multi-level outputs thanks to the usage of new design techniques.

The multi-level solution has the advantage of reducing the quantization noise at the cost of an increased complexity of the ADCs.

For this reason the output bits of these converters are mainly in the range of one (2 levels) to 5 (32 levels) and more rarely they go beyond these numbers.

Figure 1 shows a multi-level second-order sigma- delta converter 100 of the prior art in which the shown quantizer 101 is L-levels.

The converter 100 of fig. 1 is arranged to convert an input analog signal X into a stream of digital words Y.

The converter 100 includes a direct path dl having a first analog integrator 102 and a second analog integrator 103 connected in series one another upstream the quantizer 101. The converter 100 further comprises a feedback path fl arranged to subtract the digital output signal Y to the input of the first analog converter 102 and the second analog converter 103, respectively.

As known, the quantizer must not introduce delay in the direct path because the delay can cause instability, so the preferred solution to implement the quantizer is to do a flash-converter with a number of comparators equal to the output levels minus one (in this example L-l comparators) .

Other methods are possible to implement this block, but in any case low delay and a L-levels accuracy is required .

In most cases the complexity of the quantizer is the limiting factor for the increase of the number of levels. An example of multi level sigma-delta AD converter is described in the publication "A 4Ghz CT Sigma-Delta ADC with 70dB and -74dBFS THD in 125MHz BW", Bolatkale et al., pagg. 470-417, ISSCC 2011 / SESSION 27 / OVERSAMPLING CONVERTER IEEE International Solid-State Circuits Conference.

Summary of the invention

An object of the present invention is to provide a multi-level sigma-delta AD converter with reduced quantization levels which is alternative to the ones of the cited prior art overcoming at least some of their drawbacks and limitations and reducing the quantizer accuracy needed.

A multi-level sigma-delta AD converter according to the invention comprises a direct path having an input terminal to receive an input analog signal and an output terminal to provide a output digital signal corresponding to said input analog signal. The direct path comprises an analog integrator having an input terminal to receive a first analog signal representative of the input analog signal and a output terminal to provide a second analog signal, a first computation block arranged to receive said second analog signal and to provide a first analog computed signal, a quantizer having a respective input terminal connected to the first computation block to receive the first analog computed signal and a respective output terminal operatively connected to the output terminal of the direct path. The direct path further comprises a digital integrator interposed between the output terminal of the quantizer and the output terminal of the converter. The digital integrator comprises a delay block having an input terminal to receive the output digital signal and an output terminal to provide a delayed digital signal. The converter further comprises a first feedback path arranged to provide to the first computation block a feedback analog signal representative of the delayed digital signal present at the output terminal of the delay block of the digital integrator, said first computation block being arranged to subtract said feedback analog signal from the second analog signal .

An embodiment of the invention is a digital audio device comprising a multi-level sigma-delta AD converter.

Brief description of the drawings

The characteristics and the advantages of the present multi-level sigma-delta AD converter will be better understood from the following detailed description of one embodiment thereof, which is given by way of illustrative and non-limiting example with reference to the annexed drawings, in which: Fig. 1 shows a block diagram of a multi-level sigma- delta AD converter of the prior art;

Fig. 2 shows a block diagram of a multi-level sigma- delta AD converter according to an embodiment of the invention, and

Fig. 3 shows a block diagram of a digital audio device employing the multi-level sigma-delta AD converter according to the invention.

Detailed description of preferred embodiments

Block diagram of a preferred embodiment of a multi ^¬ level sigma-delta Analog-to-Digital (AD) converter of the invention can be described with reference to figure 2.

A digital audio device (described in the following with reference to figure 3) comprises the multi-level sigma-delta AD converter according to the invention.

The digital audio device can be used in any portable equipment with audio signals, e.g. mobile or cellular phone, MP3 players, PDAs (Personal Digital Assistant) , portable computers, tablets, and so on.

With reference to figure 2, the multi-level sigma- delta AD converter 200, in the following also simply converter 200, comprises a direct path dl having an input terminal Idl to receive an input analog signal X and an output terminal Odl to provide an output digital signal Y corresponding to the input analog signal X. The output digital signal Y is a digital stream of words with a number of bits depending on the number of levels of the converter 200. As an example, in the case of a 32-level sigma-delta AD converter, the output digital signal Y is a digital stream of words with 5 bits .

The direct path dl of the converter 200 comprises an analog integrator 2, e.g. a first order analog integrator, having an input terminal 12 receive a first analog signal X2 and an output terminal 02 to provide a third analog signal X3.

As an example of internal block diagram, the analog integrator 2 comprises a respective direct path comprising an adder block and a delay block connected in series one another. The adder block has an input terminal corresponding to the input terminal 12 of the analog integrator 2 and an output terminal connected to the input terminal of the delay block. The delay block has an output terminal corresponding to the output terminal 02 of the analog integrator 2. The analog integrator further comprises a respective feedback path to provide to a further input terminal of the adder block the third analog signal X3 present at the output terminal 02 of the analog integrator 2.

In addition, the direct path dl of the converter 200 comprises a first computation block S3, e.g. an adder, arranged to receive the second analog signal X3 and to provide a first analog computed signal X4.

Furthermore, the direct path dl of the converter 200 comprises a quantizer 3 having a respective input terminal 13 connected to the first computation block S3 to receive the first analog computed signal X4 and a respective output terminal 03 operatively connected to the output terminal Odl of the direct path dl .

It should be noted that, obviously, the portion of the direct path dl of the converter 200 before the quantizer 3 is in the analog domain and the portion of the direct path dl of the converter 200 after the quantizer 3 is in the digital domain.

Particularly, the quantizer 3 has a reduced number of quantization levels, e.g. 8 quantization levels, depending on the architecture of the converter 200, as it will be explained in the following.

In particular, according to the invention, the converter 200 is of the L-levels sigma-delta AD type (e.g., L=32) with a quantizer with r-levels (r<L, e.g. r=8) . An example of quantizer 3 is a flash converter having a number of comparators equal to r-1.

With reference again to the converter 200, the direct path dl further comprises a digital integrator 4 interposed between the output terminal 03 of the quantizer 3 and the output terminal Odl of the converter 200.

The digital integrator 4 is arranged to receive a digital signal DS present at the output terminal 03 of the quantizer 3 and to provide the corresponding output digital signal Y on the output terminal Odl of the direct path dl of the converter 200.

In addition, the digital integrator 4 comprises a delay block 5 having an input terminal 15 to receive the output digital signal Y and an output terminal 05 to provide a delayed digital signal DS1.

As an example of its internal block diagram, the digital integrator 4 comprises a respective direct path comprising an adder block. The adder block has an input terminal corresponding to the input terminal of the digital integrator 4 and an output terminal corresponding to the output terminal of the digital integrator 4. The digital integrator 4 further comprises a respective feedback path comprising the delay block 5 arranged to provide to a further input terminal of the adder block the delayed digital signal DS1 corresponding to the output digital signal Y of the converter 200.

With reference again to the converter 200, it advantageously comprises a first feedback path fl arranged to provide to the first computation block S3 a feedback analog signal ANl representative of the delayed digital signal DS1 present at the output terminal 05 of the delay block 5 of the digital integrator 4.

In addition, the first computation block S3 is arranged to subtract said feedback analog signal ANl from the second analog signal X3.

In more detail, in order to do so, the first computation block S3 is preferably configured to change from positive to negative the sign of the feedback analog signal ANl received from the first feedback path fl of the converter 200.

In order to convert the delayed digital signal DS1 present at the output terminal 05 of delay block 5 of the digital integrator 4 in a corresponding analog signal to be provided to the first computation block S3, the first feedback path fl further comprises a Digital-to-Analog (DA) converter DAI, e.g. a flash DA converter, interposed between the output terminal 05 of the delay block 5 of the digital integrator 4 and the first computation block S3.

With reference again to the converter 200, the direct path dl comprises a second computation block S2 arranged to receive a third analog signal XI representative of the input analog signal X and to provide a second analog computed signal X5.

The converter 200 further comprises a second feedback path f2 arranged to provide to the second computation block S2 a further feedback analog signal AN2 representative of the output digital signal Y present at the output terminal Odl of the direct path dl .

The second computation block S2 is advantageously arranged to subtract said further feedback analog signal AN2 from the third analog signal XI.

In more detail, in order to do, the second computation block S2 is preferably configured to change from positive to negative the sign of the further feedback analog signal AN2 received from the second feedback path f2 of the converter 200.

In order to convert the output digital signal Y into the further feedback analog signal AN2 to be provided to the second computational block S2, it should be observed that also the second feedback path f2 comprises a further Digital-to-Analog (DA) converter DA2, e.g. a flash DA converter, interposed between the output terminal Odl of the direct path dl of the converter 200 and the second computational block S2.

It should be observed that the direct path dl of the converter 200 further comprises a first amplification block A2, having a respective gain factor K2, interposed between the second computation block S2 and the first computation block S3.

In the embodiment of the figure 2, the first amplification block A2 of the direct path dl is interposed between the computation block S2 and the analog integrator 2.

According to a further embodiment (not shown in the figure) , the first amplification block A2 of the direct path dl is interposed between the analog integrator 2 and the first computation block S3.

Turning back to the converter 200, the direct path dl comprises a further analog integrator 1, e.g. a first order analog integrator, having an input terminal II operatively connected to the input terminal Idl of the direct path dl and an output terminal 01 operatively connected to the second computation block S2 to provide it the third analog signal XI representative of the input analog signal X of the direct path dl .

As an example, the internal block diagram of the further analog integrator 1 is analogous to the internal block diagram of the analog integrator 2, previously described .

The direct path dl of the converter 200 further comprises a third computation block SI, e.g. an adder, arranged to receive the input analog signal X and to provide a third analog computed signal X6 to the further analog integrator 1.

The second feedback path f2 of the converter 200 is further arranged to provide the further feedback analog signal AN2 to the third computation block SI.

In addition, the third computation block SI is arranged to subtract the further feedback analog signal AN2 from the input analog signal X.

In more detail, in order to do so, the third computation block SI is preferably configured to change from positive to negative the sign of the further feedback analog signal AN2 received from second feedback path f2 of the converter 200.

With reference again to the converter 200, the direct path dl further comprises a second amplification block Al interposed between the third computation block SI and the second computation block S2. The second amplification block Al has a respective gain factor Kl .

In the embodiment of figure 2, the second amplification block Al of the direct path dl is interposed between the third computation block SI and the further analog integrator 1.

In accordance with a further embodiment (not shown in the figure) , the second amplification block Al of the direct path dl is interposed between the further analog integrator 1 and the second computation block S2.

Taking the above considerations into account, the behavior of the converter 200 of the present invention is described below, with reference to the block diagram of the embodiment of figure 2.

The input analog signal X of the multi-level sigma- delta AD converter 200 is band limited when used in particular application, e.g. in the audio field.

Therefore, there is a correlation between a sample of the output digital signal Y (N) and the previous one Y(N-l) . In other words, the difference between adjacent digital samples is small.

The inventor used this correlation to predict the next digital sample subtracting the predict digital sample present at the output of the quantizer 3 directly before the quantizer 3 so that the quantizer itself can be configured with a reduced number of quantization levels to work properly.

The quantizer 3 introduced an error Err so that the first analog computed signal X4 at the input terminal 13 of the quantizer 3 has amplitude Y-Err.

In the first feedback path fl, the previous output digital sample Y(N-l) (digital output signal DS present at the output terminal 03 of the quantizer 3) is converted in the analog domain by the DA converter DAI and then subtracted from the third analog signal X3 by the first computation block S3.

In addition, the previous output digital sample Y (N) present at the output terminal Odl of the direct path dl of the converter 200 is added back at the output terminal 03 of the quantizer 3 by the digital integrator block 4.

As a result, the first analog computed signal X4 at the input terminal 13 of the quantizer 3 can be written as follows:

Y (N) -Y (N-l) -Err= ΔΥ (N) -Err (first analog computed signal X4)

As known, the output digital signal Y of a multi ^¬ level sigma-delta DA converter can be written as a function of the input analog signal X, as follows:

Y(N)=f (X(N) )+Q(N)

wherein f is a signal transfer function and Q represents the noise shaped quantization noise.

It should be noted that in the previous equation it was considered the input analog signal X is a discrete time function X (N) but the same function can be considered as valid also in the case the input analog signal X is a continuous time function X(t) .

As a consequence of the previous equation, in the estimation of the amplitude of ΔΥ (N) , there are two main terms, as follows: ΔΥ (N) = Af (X (N) ) + AQ (N)

The first term depends on the input analog signal X and on its transfer function. The second term depends on the output quantization noise.

The first term can be small if the transfer function f is a low pass filter or if the input analog signal X is band limited, as in the case of application of the converter in the audio field.

It should be note that, in order to avoid aliasing, the input analog signal X is filtered externally or by its transfer function f and therefore the first term can be small.

The second term is the difference between two successive quantization noises, so it can be assumed that the amplitude of AQ is roughly 2Q.

Then, it should be observed that the amplitude of Q is inversely proportional to the number of output levels L of the L-level sigma-delta AD converter 200 (32 in the example of figure 2) . Therefore, for a high number L of levels the amplitude of AQ is small.

These considerations can be written as follows:

AQ=4Q0/L

wherein Q0 is the quantization noise of a converter having 2 output levels.

It should be noted that the same considerations are valid for the error Err introduced by the quantizer, and therefore :

Err = 2ErrO/L

wherein ErrO is the quantization error of 2 levels quantizer.

In conclusion, the first analog computed signal X4 present at the input terminal 13 of the quantizer 3 can be represented as follows:

Y (N) -Y (N-l) -Err= Af (X (N) ) + (4Q0+ 2ErrO)/L

If the number of output levels L is big enough the amplitude signal is mainly dependent on the input analog signal X.

In the example of figure 2, having a converter 200 with 32 output levels, it is possible to arrange a quantizer 3 having a low number of quantization levels, e.g. 8.

The same quantizer 3 having 8 quantization levels can be used also in the case of a multi-level sigma-delta AD converter having 64 output levels.

With reference now to figure 3, an example of an digital audio device 300 employing the Analog-to Digital converter according to the invention is briefly described .

The digital audio device 300 is for example a mobile telephone. Such digital audio device 300 comprises a digital audio recorder chain 301 and a digital audio player chain 311. The other electronic components of the mobile telephone 300 operatively connected to both the audio chains, are not illustrated in the figure 3.

The digital audio recorder chain 301 comprises a microphone 302.

Such digital audio recorder chain 301 further comprises a microphone pre-amplifier 303.

In addition, the digital audio recorder chain 301 comprises an Analog-to-Digital Converter 200 of the type described above with reference to any of the embodiments of the invention.

Moreover, the digital audio recorder chain 301 further comprises a digital filter 304.

The digital audio player chain 311 comprises:

a further digital filter 312;

a Digital-to-Analog Converter 313;

a transducer amplifier 314, and

a speaker 315.

The multi-level sigma-delta AD converter of the invention has the advantage that the quantizer complexity does not change with the increase of the number of output levels of the converter, as explained above.

Furthermore, the accuracy and complexity of the quantizer 3 is relaxed respect to the other multi-level sigma-delta DA converter already known because its output digital signal (converted in the corresponding analog signal) is injected, via the first feedback path fl (digital integrator 4 and DA converter DAI) and the first computation block S3, directly at the input terminal 13 of the quantizer 3.

In addition, with respect to the converter of the prior art, the converter of the invention has an alternative arrangement allowing the reduction of the complexity of the quantizer. In addition, the converter of the invention is also simple because it comprises analog integrator of the first order.