Method and System for Processing an Analog Signal

Abstract

Example embodiments relate to methods and systems for processing analog signals. One example method for processing an analog signal includes modulating the analog signal using a chopping signal with a chopping frequency f.sub.chop to generate a modulated signal. The method also includes amplifying the modulated signal to generate an amplified signal. Additionally, the method includes low-pass filtering the amplified signal to generate a filtered signal that includes at least one harmonic of the modulated signal. Further, the method includes sub-sampling the filtered signal and performing a correlated double sampling operation by subtracting samples at the chopping frequency.

Claims

1. A method for processing an analog signal comprising: modulating the analog signal using a chopping signal with a chopping frequency f.sub.chop to generate a modulated signal; amplifying the modulated signal to generate an amplified signal; low-pass filtering the amplified signal to generate a filtered signal comprising at least one harmonic of the modulated signal; and sub-sampling the filtered signal and performing a correlated double sampling operation by subtracting samples at the chopping frequency.

2. The method according to claim 1, further comprising generating a square wave of frequency f.sub.chop as the chopping signal.

3. The method according to claim 1, further comprising low-pass filtering the amplified signal with a cutoff frequency of at least 3?f.sub.chop.

4. The method according to claim 3, wherein the cutoff frequency is at least 3.5?f.sub.chop.

5. The method according to claim 1, further comprising sub-sampling the filtered signal with a sampling frequency of N?2?f.sub.chop, where N is an integer.

6. The method according to claim 1, further comprising applying sinc filtering or zero-order-hold to the samples before or after performing the correlated double sampling operation.

7. The method according to claim 1, further comprising adding a gain factor in a digital domain after performing the correlated double sampling operation.

8. A system for processing an analog signal comprising: a chopper configured to modulate the analog signal using a chopping signal with a chopping frequency f.sub.chop to generate a modulated signal; an amplifier configured to amplify the modulated signal to generate an amplified signal; a low-pass filter configured to filter the amplified signal to generate a filtered signal comprising at least one harmonic of the modulated signal; an analog-to-digital converter configured to sub-sample the filtered signal; and a correlated double sampling block configured to perform a correlated double sampling operation by subtracting samples at the chopping frequency.

9. The system according to claim 8, further comprising a signal generator configured to generate a square wave of frequency f.sub.chop as the chopping signal.

10. The system according to claim 8, wherein the low-pass filter is further configured to filter the amplified signal with a cutoff frequency of at least 3?f.sub.chop.

11. The system according to claim 10, wherein the cutoff frequency is at least 3.5?f.sub.chop.

12. The system according to claim 8, wherein the low-pass filter is further configured to settle before the sub-sampling of the filtered signal.

13. The system according to claim 8, wherein the analog-to-digital converter is further configured to sub-sample the filtered signal with a sampling frequency of N?2?f.sub.chop, where N is an integer.

14. The system according to claim 8, wherein the analog-to-digital converter is a successive approximation register analog-to-digital converter.

15. The system according to claim 8, wherein the correlated double sampling block is further configured to apply sinc filtering or zero-order-hold to the samples before or after performing the correlated double sampling operation.

16. The system according to claim 8, wherein the correlated double sampling block is further configured to add a gain factor in digital domain after performing the correlated double sampling operation.

17. The system according to claim 8, wherein the amplifier is an instrumentation amplifier, and wherein the analog signal is a sensor-generated signal.

18. The system according to claim 17, wherein the analog signal is a micro-level signal.

19. A system comprising: a sensor configured to generate an analog signal; a chopper configured to modulate the analog signal using a chopping signal with a chopping frequency f.sub.chop to generate a modulated signal; an amplifier configured to amplify the modulated signal to generate an amplified signal; a low-pass filter configured to filter the amplified signal to generate a filtered signal comprising at least one harmonic of the modulated signal; an analog-to-digital converter configured to sub-sample the filtered signal; and a correlated double sampling block configured to perform a correlated double sampling operation by subtracting samples at the chopping frequency.

20. The system according to claim 19, wherein the analog signal is a micro-level signal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Example embodiments of the present disclosure are now further explained with respect to the drawings by way of non-limiting example only. In the drawings:

[0030] FIG. 1 shows a method, according to example embodiments.

[0031] FIG. 2 shows a system, according to example embodiments.

[0032] FIG. 3 shows a correlated double sampling block of FIG. 2, according to example embodiments.

[0033] FIG. 4 shows a signal progression for the system of FIG. 2 in the frequency domain, according to example embodiments.

[0034] FIG. 5 shows sub-sampling schemes for the analog-to-digital converter of FIG. 2, according to example embodiments.

[0035] FIG. 6 shows a system, according to example embodiments.

DETAILED DESCRIPTION

[0036] Reference will now be made in detail to the embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings. However, the following embodiments of the present disclosure may be variously modified and the range of the present disclosure is not limited by the following embodiments. Reference signs for similar entities in different embodiments are partially omitted.

[0037] In FIG. 1, an example embodiment of the method 100 according to the first aspect of the disclosure is illustrated. In a first step 101, an analog signal is modulated using a chopping signal with a chopping frequency to generate a modulated signal. In a second step 102, the modulated signal is amplified to generate an amplified signal. In a third step 103, the amplified signal is low-pass filtered to generate a filtered signal comprising at least one harmonic of the modulated signal. In a fourth step 104, the filtered signal is sub-sampled and correlated double sampling operation is performed by subtracting the samples at the chopping frequency.

[0038] In FIG. 2, a first example embodiment of the system 200 according to the second aspect of the present disclosure is illustrated. The system 200 may comprise devices (not shown) for receiving an analog signal 201 (e.g., a sensor-generated signal) and may provide a sensing channel or a readout channel in order to process the analog signal 201. The devices for receiving the analog signal 201 may comprise or be a sensor readout unit or circuitry or interface, for example, a Wheatstone bridge-based readout circuitry or any straightforward sensor-readout unit, circuitry, or interface. The devices for receiving the analog signal 201 may receive the analog signal 201 through a wired connection or wirelessly.

[0039] The system 200 may further comprise a chopper 202 that may receive the analog signal 201 and may modulate the analog signal 201 using a chopping signal 212 with a chopping frequency f.sub.chop to generate a modulated signal 203. In this regard, the term chopper should be understood as a multiplier or a modulator that may multiply or mix an input signal with a carrier signal in order to up-modulate or to shift the input signal from the baseband to the frequency of the carrier signal in the frequency spectrum including the generation of corresponding signal harmonics, e.g., due to the modulation with a square wave chopping signal.

[0040] The system 200 may further comprise an amplifier 204 operably coupled to the chopper 202. The amplifier 204 may receive the modulated signal 203 at one or more inputs of the amplifier 204 and may amplify the modulated signal 203 to generate an amplified signal 205. The amplifier 204 may be a band-limited amplifier having a bandwidth of about 35?f.sub.chop. Alternatively, the amplifier 204 may be designed to have a bandwidth based on the gain accuracy, e.g., a bandwidth lower than 35?f.sub.chop that may increase the gain error. The amplifier 204 may introduce input-referred amplifier DC offset and low-frequency noise components, hereinafter referred as noise signal, while amplifying the modulated signal 203. The noise signal introduced by the amplifier 204 may remain at the baseband or at DC in the frequency spectrum.

[0041] The system 200 may further comprise a low-pass filter (LPF) 206 operably coupled to the amplifier 204. The LPF 206 may receive the amplifier signal 205 and may filter the amplified signal 205 to generate a filtered signal 207. The LPF 206 may have a cut-off frequency of about 3.5?f.sub.chop. As such, the filtered signal 207 may comprise one or more harmonic components or chopping harmonics present in the frequency spectrum due to the chopping operation.

[0042] For instance, the chopping signal 212 may be a square wave of frequency f.sub.chop. As such, the modulated signal 203 generated by the chopper 202 may comprise the analog signal 201 at the chopping frequency f.sub.chop, a first (odd) harmonic component at the frequency 3?f.sub.chop, a second (odd) harmonic component at the frequency 5?f.sub.chop and so on in the frequency spectrum. As the LPF 206 may filter the amplified signal 205 with a cut-off frequency of about 3.5?f.sub.chop, the filtered signal 207 may comprise the noise signal at the baseband, the analog signal 201 at the chopping frequency f.sub.chop, and the harmonic component at the frequency 3?f.sub.chop.

[0043] The system may further comprise an analog-to-digital converter (ADC) 208 operably coupled to the LPF 206. The ADC 208 may receive the filtered signal 207 and may perform sub-sampling of the filtered signal 207 to generate discrete samples 209. The ADC 208 may sub-sample the filtered signal 207 with a sampling frequency of N?2?f.sub.chop, e.g., with a sampling frequency of 2?f.sub.chop for sampling twice during one chopping period T.sub.chop.

[0044] The system may further comprise a correlated double sampling (CDS) block 210 operably coupled to the ADC 208. The CDS block 210 may operate at the chopping frequency f.sub.chop, may receive the samples 209 from the ADC 208, and may perform correlated double sampling operation by subtracting the samples 209 at the chopping frequency f.sub.chop. For example, the ADC 208 may provide two samples corresponding to two opposite signal level instances or successive levels. The CDS block 210 may subtract the samples, which may eliminate the noise signal, however, the useful signal may be doubled in amplitude.

[0045] The system may further comprise a signal generator 211 operably coupled to the chopper 202, the ADC 208, and the CDS block 210. The signal generator 211 may generate the chopping signal 212 and may provide the chopping signal 212 to the chopper 202. The signal generator 211 may further generate the sub-sampling clock 213 and may provide the sub-sampling clock 213 to the ADC 208. The signal generator may further generate the CDS clock 214 and may provide the CDS clock 214 to the CDS block 210.

[0046] It is to be noted that the coupling between the above-mentioned entities (e.g., between the entities including the chopper 202, the amplifier 204, the LPF 206, and the ADC 208) may correspond to a single-ended connection or a differential connection.

[0047] In FIG. 3, an example embodiment of the CDS block 210 is illustrated. The CDS block 210 may comprise a first decimation block 301 that may receive the ADC samples 209 with a sampling frequency of N?2?f.sub.chop and may decimate or down-sample the samples 209 by N samples, thereby resulting the decimated samples 302 at a sampling frequency of 2?f.sub.chop. In this regard, the first decimation block 301 may perform a zero-order hold operation or sinc filtering to attenuate the ADC samples 209.

[0048] The CDS block 210 may further comprise a subtraction block 303 operably coupled to the first decimation block 301. The subtraction block 303 may operate at the chopping frequency f.sub.chop, may receive the decimated samples 302 from the first decimation block 301, and may subtract the samples 302 at the chopping frequency f.sub.chop to generate subtracted samples 304.

[0049] The CDS block 210 may further comprise a second decimation block 305 operably coupled to the subtraction block 303. The second decimation block 305 may receive the subtracted samples 304 and may decimate or down-sample the samples 304 by 2 samples, thereby resulting the further decimated samples 306 at a sampling frequency of f.sub.chop. In this regard, the second decimation block 305 may perform a zero-order hold operation or sinc filtering to attenuate the subtracted samples 304.

[0050] The CDS block 210 may further comprise a third decimation block 307 operably coupled to the second decimation block 305. The third decimation block 307 may receive the further decimated samples 306 and may decimate or down-sample the samples 304 to generate the output discrete signal 308 at a suitable frequency, e.g., 40 Hz. In this regard, the third decimation block 307 may perform a zero-order hold operation or sinc filtering to attenuate the further decimated samples 306.

[0051] In FIG. 4, an example signal progression for the system 200 (e.g., especially in the frequency domain) is illustrated. The signal 201 may correspond to the analog signal 201 comprising the frequency components 400 at the baseband. The signal 203/205 may correspond to the chopper output 203 or the amplifier output 205. The noise components 401 and 402 may correspond to the combination of the input-referred amplifier DC offset and low-frequency noise from the amplifier 204. The original baseband frequency components of the noisy modulated signal 203/205 are modulated and are shown at the odd harmonics, e.g., the frequency component 400 at f.sub.chop, the frequency component 403 at 3?f.sub.chop, the frequency component 405 at 5?f.sub.chop, and so on.

[0052] The signal 207 may correspond to the filter output, i.e., the ADC input 207, where the LPF 206 may apply a cut-off frequency of 3.5?f.sub.chop, as shown by the filter band 406 (e.g., to include the noise components 401, 402 at the baseband), the frequency component 400 at f.sub.chop, and the frequency component 403 at 3?f.sub.chop.

[0053] For example, the chopper 202 may up-modulate the analog signal 201 A*cos(?.sub.int), where A is the signal amplitude and win is the signal frequency, e.g., with a square wave of +1 and ?1. The Fourier expansion of the chopping signal 212 can be expressed as:

[00001]$\frac{4}{?}\left\{\sin \left({?}_{\mathrm{chop}}t\right)+\frac{1}{3}\sin \left(3{?}_{\mathrm{chip}}t\right)+\frac{1}{5}\sin \left(5{?}_{\mathrm{chop}}t\right)+.Math.\right\}$

where ?.sub.chop is the chopping frequency.

[0054] The first term of the modulated signal 203 can be expressed as:

[00002]$\frac{4}{?}*A*\left\{\frac{1}{2}\sin \left(\left({?}_{\mathrm{chip}}-{?}_{\mathrm{in}}\right)t\right)-\frac{1}{2}\sin \left(\left({?}_{\mathrm{chop}}+{?}_{\mathrm{in}}\right)t\right)\right\}$

[0055] It can be understood that the amplitude

[00003]$\frac{4}{?}*A$

A is greater than the original amplitude, and therefore, a gain error may arise.

[0056] The signal 209 may correspond to the ADC output 209, where the ADC 208 may operate at a sampling frequency of 2?f.sub.chop. In some embodiments, the ADC 208 may sub-sample all signals greater than f.sub.s/2, which may result in the amplitude A due to the sub-sampling of the frequency component 400 at f.sub.chop and the frequency component 403 at 3?f.sub.chop, as shown by the frequency component 407, thereby reducing the gain error. Furthermore, the ADC 208 may sub-sample the noise components, e.g., the noise components at 2?f.sub.chop to the baseband. Moreover, in order to eliminate aliasing, the LPF 206 may settle before the ADC 208 takes samples.

[0057] The signal 306 may correspond to the CDS block 210 output signal 306 of FIG. 3. The CDS block 210 may subtract the ADC samples at f.sub.chop to demodulate the frequency components 407 back to the baseband, as shown by the frequency component 408 (e.g., by providing band-pass filtration at f.sub.chop). The noise component 401 at the baseband can be eliminated because of the high-pass filtration at or around f.sub.chop or f.sub.s/2, and the noise component 402 can be eliminated or minimized by sample subtraction. The effective noise components at the CDS block 210 output are shown as the noise components 409 and 410, e.g., the noise component 409 may correspond to the attenuated DC offset of the amplifier 204.

[0058] In FIG. 5, example sub-sampling schemes for the ADC 208 are illustrated. For example, the ADC 208 may sub-sample the filtered signal 207 with a sampling frequency of 2?f.sub.chop, i.e., N=1. In this regard, the ADC 208 may take samples at the edges of the successive signal levels (top left figure). Accordingly, the CDS block 210 may subtract the two successive samples, e.g., with a zero-order hold at a sampling frequency of f.sub.chop.

[0059] As a result of the subtraction and due to the constant level of the noise signal floor, the noise signal can be reduced, while at the same time due to the opposite nature of the actual signal component, the resultant signal may be doubled in amplitude. However, in this example (e.g., due to the sub-sampling of the noise components because of the band-limited anti-aliasing filtering of the LPF 206), the signal-to-noise ratio (SNR) may remain the same.

[0060] Alternatively, the ADC 208 may sub-sample the filtered signal 207 with a sampling frequency of 4?f.sub.chop, i.e., N=2. In this regard, the ADC 208 may take samples at the edges of the successive signal levels (top right figure).

[0061] Further alternatively, the ADC 208 may sub-sample the filtered signal 207 with a sampling frequency of 4?f.sub.chop, i.e., N=2. In this regard, the ADC 208 may take samples at different instances with a defined sampling span on the successive signal levels (bottom left figure).

[0062] Further alternatively, the ADC 208 may sub-sample the filtered signal 207 with a sampling frequency of 8?f.sub.chop, i.e., N=4. In this regard, the ADC 208 may take samples at the edges of the successive signal levels (bottom right figure) or may take samples at different instances with a defined sampling span on the successive signal levels (not shown).

[0063] In FIG. 6, a second example embodiment of the system 600 according to the second aspect of the present disclosure is illustrated. The system 600 may correspond to the system 200, e.g., the system 600 may comprise the chopper 202, the amplifier 204, the LPF 206, the ADC 208, the CDS block 210, and the signal generator 211. The system 600 may differ from the system 200 in that the system 600 may additionally comprise a gain block 601 (e.g., coupled to the CDS block 210) in order to apply a digital gain at the CDS block 210 output.

[0064] In this regard, the system 600 may omit the LPF 206, and the digital gain provided by the gain block 601 may be defined to reduce the gain error to improve the gain accuracy. Alternatively, the system 600 may include both the LPF 206 and the gain block 601 to further reduce the gain error, thereby further improving the gain accuracy.

[0065] It is important to note that, in the claims as well as in the description, the word comprising does not exclude other elements or steps and the indefinite article a or an does not exclude a plurality. A single element or other unit may fulfill the functions of several entities or items recited in the claims. Furthermore, the word coupled implies that the elements may be directly connected together or may be coupled through one or more intervening elements. Moreover, the description with regard to any of the aspects is also relevant with regard to the other aspects of the present disclosure.

[0066] Although the present disclosure has been illustrated and described with respect to one or more implementations, equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In addition, while a particular feature of the present disclosure may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired for any given or particular application.