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A CMOS Chopper Amplifier IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. SC-22, NO. 3, JUNE 1987 335 A CMOS Chopper Amplifier CHRISTIAN C. ENZ, STUDENT MEMBER, IEEE, ERIC A. VITTOZ, MEMBER, IEEE, AND FRAN~OIS KRUMMENACHER Abstracf —This paper describes a highly sensitive CMOS chopper Section II will review different low-frequency noise and amplifier for low-frequency applications. It is realized with a second-order offset reduction techniques, emphasizing the fundamental low-pass selective amplifier using a continuous-time filtering technique. differences between the autozero and chopper techniques. The circuit has been integrated in a 3-pm p-well CMOS technology. The chopper amplifier dc gain is 38 dB with a 2@-Efz bandwidth. The equiv- Section III will present the realization of the selective alent inpnt noise is 63 nV/dHz and free from l/~ noise. The input offset amplifier, whereas the theoretical performances of the is below 5 VV for a toning error less than 1 percent. The amplifier chopper amplifier will be discussed in Section IV. Finally consumes only 34 ~W. the circuit implementation and the experimental results will be presented in Sections V and VI. 1. INTRODUCTION II. LOW-FREQUENCY NOISE AND OFFSET N SPITE OF the evolution of scaled-down digital REDUCTION TECHNIQUES [3] Iprocesses, analog circuits will always be needed to per- form a variety of critical tasks required to interface the 4. Input Device Optimization digital with the external world. One of these functions is high-precision amplification with low power consumption. The classical approach to reduce the l/f noise is to The amplifier must be fully compatible with a process enlarge the gate area of the input devices [3]. This method basically tailored for digital requirements like the CMOS is area costly and inefficient because offset and l/f noise technology. subsist at low frequency. A second approach is to use It is well known that highly sensitive CMOS amplifiers MOS input transistors operated in the lateral bipolar mode are always limited by offset and I/j noise. An offset [4], [5]. This mode of operation provides a reduction of voltage of 10 mV and a comer frequency of 10 kHz are more than 40 dB for the l/f noise with an offset typically typical values for a CMOS amplifier. A combination of comprised between 1 and 10 mV. Further improvements of autozero and chopper techniques was used by Poujois and the l/f noise and the offset require special circuit tech- Borel [1] to realize a fully integrated MOS amplifier with niques. typical offset of 5 pV and an equivalent input noise of 2.5 pV/~Hz. However, two external capacitors were required B. A utozero Technique and the amplifier consumed 240 mW! Good noise perfor- mance was obtained by Hsieh et al. [2] in a CMOS The principle of the autozero technique is represented in differential chopper amplifier for SC filter applications. Fig. 1. In the sampling phase @l the output and the input The equivalent input noise was 40 nV/~Hz for a chopper of the amplifier are shorted together, so that the input frequency of 128 kHz. The amplifier displayed a 15-MHz noise is sampled in capacitor C. In the amplification phase gain-bmdwidth product and 4-rnW power dissipation with @2 the noise sample is subtracted from the instantaneous k 7.5-V supplies. noise of the amplifier. Since the sampled noise and the This paper presents a highly sensitive CMOS chopper low-frequency continuous noise are highly correlated dur- amplifier realized with a second-order low-pass selective ing a sampling period, the l/f noise is removed. “Thesame amplifier. The objective is to reach the microvolt level for is true for the amplifier offset, except that offset will both offset and noise, while keeping the total power con- subsist due to charge injection. In the case of a–single-pole sumption below 100 ILW.The bandwidth is then limited to amplifier, the transfer function is given by a few hundred hertz by the fundamental thermal noise. AO A(f)=— (1) Manuscript received October 14, 1986,; revised January 5, 1987, This l+j~ work was supported by the Fends National Suisse pour la Recherche Jc Scientifique,.PN13. C. C. Enz and F. Krummenacher are with the Electronics Laboratory, where AO is the dc gain and f, the cutoff frequency. The Swiss Federrd Institute of Technology-. (EPFL), CH-1OO7 Lausanne, equivalent amplifier input noise can be expressed as’ Switzerland. E. A. Vittoz is with the Centre Suisse d’Electronique et de wcrotech- fk nique S.A. (CSEM,), CH-2000 Neuchiltel 7, Switzerland. (2) IEEE Log Number 8713955. ‘N’”= ‘“0 ()1+ m 0018-9200/87/0600-0335 $01.00 01987 IEEE / 336 IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. SC-22, NO. 3, JUNE 1987 m,(t) ~ ,=,,2 r----% m,(t) t m2(t ) * 9 9 -y+++ vi 1 “ o J LfTLfTkfTkfT Fig. 1. Autozero amplification principle Fig. 3. Chopper amplification principle. However, the output noise is dominated by the amplifier undersampled w~te noise. Thus the l/f noise is removed at the cost of an increase of the white-noise component by -1 /0 a factor ~fCT,. The offset cancellation is limited by charge injection due to the autozero switch. It can be reduced to a mismatch of charge injection by using a fully differential implementa- tion. The basic autozero principle described above can be improved by storing the offset and the noise information at the output of a first amplifier stage [8]. Effects due to . .. ~ o 1 2 3 4 5 charge injection and noise undersampling are thus reduced by a factor proportional to the gain of this first amplifier. fT, However, the need of a two-stage amplifier gives rise to Fig. 2. Baseband transfer function lHO(~) 1. potential stability problems. Another possibility & to use an amplifier with a low-sensitivity auxiliary input con- where S~Ois the amplifier white-noise component and f~ trolled by the compensating voltage [9], [10]. the corner frequency. Assuming that AO >>1, fCT, >>1, The alternative to the autozero method is the chopper f~~ < 1/2, and (~1/~2) <<1, the avera~e output noise technique, which will be described in the next section. power spectrum is given by [6] C. Chopper Technique [2] fk Sivo.t(.f) ‘4%0 1+ fi l~o(.f)l’ The chopper amplification principle is illustrated in Fig. (( ) 3. Suppose that the input signal has a spectrum limited to + nfc~sine’ ( nf T, ) (3) fChOP/2and that the amplifier has neither noise nor offset. } This input signal is multiplied by the square-wave modulat- where ion signal ml(t) with period T= l/fChOP.After this mod- ulation, ‘the signal is transposed around the odd harmonic fr~quencies of the modulation signal. It is then amplified l~o(f)l’ =(l-sinc(mfl,))’+ and demodulated back to the original band. Because of the (1-’%7)2 ‘4) finite bandwidth of the amplifier, the output signal con- (5) tains spectral components aroun,d the even harmonics of the chopper frequency. The bilateral Fourier transform (BFT) of the output voltage Uo(t) is given by The first term in (3) represents the baseband noise attenua- tion, while the second term comes from the undersampling of the amplifier white noise [7]. The transfer function Vo(f) = y Hk(f”)~ f –,; (7) kk=e;ec l~o(f )1 is represented in Fig. 2. For ~fl$ <1, it can be (1 approximated by a differentiator function where ~fls 2 l~o(f)12= y . (6) () Because of the double zero introduced by the baseband transfer function [HO(f) 12, the l/f noise is removed. for k even (8) ENZ d a[.: CMOS CHOPPER AMPLIFIER 337 +Vinj t ‘Vinj T I!zfT (a) . infinite bandwidth 2 ~ t E ideal low-pass a A 103 , , 10= m % ~ fT 1 3 579 11 (b) z Fig. 5. (a) Spikes’ signal (b) Spikes’ signat and modulated signaf spec- Yi tra with amplifier transfer function characteristics. g 10 10 tively, the chopper amplifier schematic and the measured 1 10 10= 103 10’ equivalent input noise. The measured input white noise Froquoncy [Hz] without the chopper is 37 nV/~Hz and the theoretical input white noise given by (11) is 50.5 nV/~Hz for &OP= (b) f~ = 1 kHz, which is very close to the measured result. Fig. 4. (a) Expenm@af chopper amplifier schematic. 0,41 and OA 2 were, respectively, a CA 3420 and a pA 741 and the switches were Equation (11) shows that if f~ >>fCkOPthe equivalent MC 14016. ~chOP= fk=1 klfz. (b) Measured equivalent input noise. low-frequency input noise of the chopper amplifier is equal to the original amplifier white-noise component. Contrary ~.(~) is the BFT of the input voltage, and tO is a possible to the autozero techmque, the white noise is not aliased delay between the input and output modulation signals to because there is no sample-and-hold process. This suggests compensate for the phase shift introduced by the amplifier. that the autozero technique is more suitable for sampled- After low-pass filtering at &OP/2, (7) becomes data circuits like SC filters [11] where the undersampling process is unavoidable and that the chopper technique is vo[f)=Ho(f)fi(f) (9) better used in continuous:time applications. The high-frequency components, which also include the where HO(~) represents the baseband transfer function amplifier l/f noise and offset, can easily be removed by a and is given by (8) for k = O. simple low-pass fiiter. The noise and the offset of the amplifier are only As is the case for the autozcwo technique, the offset of modulated once and translated tb the odd harmonics of the chopper amplifier is lirriited by charge injection r&- the output chopping square wave.
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