Nevertheless it has been believed [I] that, in contrast with AZCs, both DEMCs and ... In comparison with Waditional circuits using dynamic element matching and ...
HIGH-ACCURACY INSTRUMENTATION AMPLIFIER FOR LOW VOLTAGE LOW POWER CMOS SMART SENSORS Christian Falconi', Marco Faccio', Arnaldo D'Amico'.Corrado Di Natale' 2
'
Dept. of Electronic Engineering, University of Tor Vergata, Roma, Italy Dept. of Electrical Engineering, University of L'Aquila, Monteluco di Roio, L'Aquila, Italy
ABSTRACT In low voltage low power CMOS systems both the high input offset and noise voltages of MOSFETs and also the moderate gain of op amps limit the accuracy of analog circuits. In this paper we present a new high-accuracy instrumentation amplifier; this circuit topology is the first ever reported "non-autozero" instrumentation amplifier which allows to compensate, beside the input offset and noise voltages of the op amps, also the finite op amp gain; furthermore proper dynamic resistor matching may improve both the accuracy of the closed loop gain and the CMRR.
1. INTRODUCTION Smart sensors contain both the sensing elements and the electronic
interfaces in the same chip, thus having many potential advantages such as low cost, improved accuracy and reliability, low power consumption, better rejection against interferences, reduced size and weight, ... In most cases it is necessary (mainly for low cost purpose) to integrate smart sensors in CMOS or in CMOS compatible processes, so that the implementation of high-accuracy electronic interfaces is problematic. In fact, on one hand accurate analog signal processing typically requires operational amplifiers (op amps) with very high gain and with low input offset and noise signals; on the other hand, CMOS amplifiers generally have high input offset and noise voltages and relatively small gain per stage; these non-idealities are even worse in low voltage low power systems (low current consumption leads to higher noise levels, Limited headroom makes difficult to use cascading techniques,...). As a result, in smart sensors design it is often mandatory to compensate the non idealities of CMOS op amps; since some of these (significant) non idealities, for instance the noise, induce time-varying errors, "static" compensation techniques are of little use. CMOS circuits using "automatic" (or "dynamic") techniques [1,2,3,4,5,6,7,8] for the compensation of errors introduced by non idealities of amplifiers can be grouped in three main classes: autozero (switched capacitors) circuits (AZCs), chopper circuits (CHCs) and circuits using the dynamic element matching (DEMCs). In the classification generally accepted in literature [1,2,3,4,5] DEMCs and CHCs are grouped in the same class; however, as we have already discussed elsewhere [9,10,11], it is bener to distinguish between those two classes (although there are circuits which belong to both the DEMCs and the CHCs classes, some other circuits belong to one class but not to the other one). In particular we have recently introduced [9,10,1 I] circuits based on dynamic matching of op amps (DOAMCs); DOAMCs are a subset of DEMCs, but they may not be regarded as CHCs (in particular their accuracy and noise properties are substantially different from those ofCHCs [9,10]).
0-7803-7761-3/03/$17.00 02003 IEEE
Although DEMCs and CHCs are two different classes of circuits, they do have some similarities; in particular DEMCs and CHCs typically have better noise performance than AZCs [ I ] because they do not require sampling (sampling inevitably means undersampling the wide-hand thermal noise [I]). Nevertheless it has been believed [I] that, in contrast with AZCs, both DEMCs and CHCs may not achieve gain enhancement (i.e. compensation of the finite op amp gain). As a consequence, in those applications where the gain of op amps is severely limited by low voltage low power specifications, autozeroing has been generally preferred. On the contrary we have shown [9,10,11] that DOAMCs (which are a subset of DEMCs) may achieve a strong gain enhancement while having better noise properties (and thus, potentially, better accuracy) than AZCs. In comparison with Waditional circuits using dynamic element matching and with chopper circuits, DOAMCs may achieve a strong gain enhancement, but at the cost of higher noise (the equivalent input noise m voltage is typically fitimes higher), larger area (about doubled) and power consumption (about doubled); these costs could be affordable in many low voltage low power systems for the vely large gain enhancement that may result from dynamic op amp matching. In contrast with gain enhanced AZCs, DOAMCs have a lower input residual noise and do not require high value linear capacitors, which is an important advantage for integration in digital CMOS processes [12,13]; on the other hand DOAMCs require, for low power consumption, relatively high resistor values which may not be integrated in those digital CMOS processes where silicide is used to reduce the sheet resistances of polysilicon and of diffusion layers [13,14,15] (this technological limit may be removed, at some cost, by using an additional mask to selectively black the deposition of silicide on regions needed for the integration of resistors; in some cases, well resistors could be used but their voltage dependence [IS] should be considered). We also mention that, in principle, the resistors contained in DOAMCs introduce thermal noise; nevertheless in most cases the residual noise of CMOS circuits (even after compensation of the low frequency noise) is dominated by the noise generated by MOSFETs.
The dynamic op amp matching technique might also be applied to the replica-amplifier circuits presented in [16,17] (in those circuits the gain enhancement was limited by gain mismatch), although the simple dynamic op amp matching would not ContemporUly lead, in those circuits, to the reduction of the equivalent input offset and low frequency noise voltages. Since many smart sensors applications require a very small bandwidth (in some cases a few H e m [Z]),the power consumption
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of DOAMCs m y be significantly reduced by powering down the correspondent circuitry for large amounts of time. In this paper we present an insmentation amplifier based on the
dynamic op amp matching. The proposed circuit may compensate (without autazeroing, and therefore with hener noise performance) the input offset and noise voltages and the finite gain of the op amps; furthermore, if proper dynamic element matching is also applied to the resistors, both the accuracy of the closed loop gain and the CMRR of the instrumentation amplifier may be substantially improved (this topic is extensively discussed in [7]).
2. DOAM INSTRUMENTATION AMPLIFIERS
Figure 3 shows the first stage of a DOAM instrumentation amplifier (switches are not shown in figure); if the resistances RA, Rs and Rc satisfy the design rules
Rc = R RA = RB = 2Rc = 2R
(3)
and ifwe define the ideal voltage gain
we may find for the output voltage error (ideal matching of the op amps)
E,"# = v, - 4(PI - u 2 ) =
2.1 Dynamic op amp matching Under the unrealistic hypothesis of ideal matching among different op amps, it is possible to design circuits where the errors introduced by the different op amps cancel each other; in practice the ideal matching hypothesis may be removed if the dynamic element matching technique is applied to the op amps.
2.2 Instrumentation amplifiers
Comparing the errors (2) and ( 5 ) we notice that the DOAM circuit achieves a strong gain enhancement (the output voltage error is now, approximately, inversely proportional to d rather than to GI.
In order to evaluate the errors of circuits containing op amps we can use the following simplified (for instance we neglect the output impedance, the sensitivity to power supply, ...) linear model
where C is the differential open loop gain of the op amp, GCMis the common mode gain, V+and V. are the voltages at the positive and at the negative input terminals, V,fiou, is the output offset voltage; for simplicity we assume all other components (but op amps) ideal and we indicate the output voltage error (that is the difference between the output voltage and the ideal output voltage) as En,,f. In the following analytical relations we do not explicitly consider the input low frequency noise voltage; nevertheless, in a short time interval, T,, the input noise voltages with frequencies much smaller thanf, = /&are about constant, so that, within that short time interval, those noise voltages may be regarded as additional input offset voltages [1,2] (in practice Txis the time required for the single dynamic compensation operation).
-"" ' "
Figure I. First stage of the "three op amps" instrumentation amplifier
The classic "three op amps" insmentation amplifier is constituted by a first fully differential stage (figure I ) followed by a differential amplifier (figure 2). Although the second stage (i.e. the differential amplifier) is less critical because its input signal has yet been amplified by the first stage, in many cases it may be necessary to counteract even its errors. For this reason we present the DOAM versions of both the first stage and the second stage.
DOAM instrumentation umphfier: first s t u p In the case of the classic instrumentation amplifier (figure I), using the linear model ( I ) for the op amps, defining the ideal voltage gain A, := (2Ri + R , ) / R , , and considering identical op amps we
f
find for the output voltage error
Em,= 5°C
-
=
2GcM-2A, 2G - G,
+ 2Av
A Y ( v , - v 2 ) (2)
Figure 2. Second stage of the "three op amps" instrumentation amplifier (differential amplifier)
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The previous relations have been obtained by considering identical op amps, which is not the case in practice; in [9] we have theoretically shown that the mismatch among practical op amps is strongly reduced by applying the dynamic element matching to the op amps; both SPICE simulations and experimental results on the effectiveness of the dynamic op amp matching in compensating the mismatch ofpractical op amps have been presented in [9,10]). We stress that it is useful to contemporarily apply the dynamic element matching also to the resistors in order to improve both the accuracy of the closed loop gain and the CMRR (see [7]).
Since for good accuracy the design NICS (3) must be accurately satisfied and the ideal voltage gain (4) must also be accurate, all the resistors R,. R>, RA, RB and R , must be of the same type and properly laid out.
DOAM instrumenfalionamplifier: second stage
In the case of the classic differential amplifier (figure 2), assuming again the linear model (I) and defining the ideal voltage gain A, := R2/ R, we find for the output voltage error
E"", = v,,. - A" ( 3 - VI 1=
+2C+2A,-GcM+2( "CM
+
AV
2(A" + I ) 2G + 2 A, - G,
VI + V I
2
]
-t
+ 2 '"' "'"'
In the case of the DOAM differential amplifier shown in figure 4, if the compensation resistances RA, RB, R , and RD are sized according to the design NICS
R,=R,=ZR R, = RD = R
(7)
and if we define the ideal voltage gain A, := ( R , + R ) / R, , we find for the output voltage error (ideal op amp matching)
- A" ( v , - v2 ) = - -2A, [G& + 4 - 4Gc, + 4A, ( 2 - 4Gc, + A,)] -
E"", = V". Figure 3. First stage of the DOAM instrumentation amplifier (the op amps must be dynamically interchanged by switches not shown in the figure)
d
(VI
-.*I+
+ - ~ [ ~ A $ c M- 4 A v G c ~+2G&Av] d
+
-2[-4A~+2A,GCM-8A,-4+2G,,] """,W
d
d =4G' +G[8( Ay +1) - 4 G c M ] + +8Ai
+ 1 6 4 + GfM- G,
[ 8 ( A, + I ) ]
+8
(8) The comparison between the output voltage errors of the classic differential amplifier (6) and of the DOAM differential amplifier (8) shows that, even in this case, the dynamic op amp matching allows to achieve a strong gain enhancement.
2.3 Implementation of DOAMCs
IRO Figure 4. Second stage of the DOAM instrumentation amplifier (the op amps must be dynamically interchanged by switches not shown in the figure); this circuit may also be used as a DOAM differential amplifier
Two op amps may be dymnically matched by using chopper switches as shown in figure 5 (chopper switches are two ports networks constituted by switches connected in such a way that the "straight" connections are enabled in a first phase and the "cross" connections are enabled in the second phase). In principle, the parasitic resistances of the CMOS switches (required for the dynamic op amp matching) do not introduce
significant errors since those switches are in series with the input terminals of the op amps (and thus negligible currents flow across such switches) or they are in series with the output ofthe op amps and therefore (if the switches are properly placed) the errors due to parasitic resistances are lowered by the feedback. In the DOAM i n s m e n t a t i o n amplifier (first stage and second stage) there are six op amps; clearly the matching between op amps is only important for op amps belonging to the same stage; in order to dynamically interchange the four op amps of the first stage a four phases clock is required. The analysis of the stability of DOAMCs is apparently complex because of the presence of several feedhack loops; however simple star-mangle transformations greatly simplify the analysis of the stability and, eventually, the frequency compensation. Finally we report the results of same SPICE simulations; for simplicity we restrict our analysis to the first stages of both the classic and the DOAM instrumentation amplifiers. Since, of course, proper dynamic element matching may compensate the errors introduced by offset of op amps, we restrict our attention to the compensation of the finite gain of op amps and, therefore, we assume that the op amps are ideal except for their finite differential gains. The mismatch between the differential gains of nominally identical op amps integrated within the same chip (experiencing about the same temperature, supplied by the same voltages, ...) may typically be reduced down to i l O % in normal operative conditions. For this reason we considered, for the DOAM circuit shown in figure 3, four op amps having as the differential gains 9000, 9500, 10500 and llOW; the simulated relative gain error ufler the dynamic op amp matching. was about 3ppm (in the classic circuit in figure 1, if we consider two op amps having a differential gain equal to 10000, the relative gain error is 1000ppm); if the differential gains of all the op amps are reduced by a factor IO the relative gain error is 9900ppm in the classic circuit and 2lOppm in the DOAM circuit (aflw the dynamic op amp mulching).
CH2
Figure 5. Chopper switches for the dynamic matching of two op amps
3. CONCLUSIONS In low voltage low power CMOS smart sensors it is often necessary to reduce both the errors due to the input offset and noise voltages of op amps and the errors due to their finite gains. Although autozeroing has been considered the only possible solution to this problem, we have recently introduced an alternative approach, the dynamic op amp matching. In comparison with gain enhanced autozero circuits, the circuits using dynamic op amp matching (DOAMCs) may achieve superior accuracy because
they do not require sampling and, therefore, their residual noise is not deteriorated by the undersampling of the wide-band thermal noise. We also stress that DOAMCs do not require the integration of high value linear capacitances (although they do need relatively large resistances). In this paper we have presented a DOAM instrumentation amplifier; the proposed circuit is constituted by two distinct stages; for each of these stages it is possible to achieve a strong gain enhancement, under the condition that the compensation resistances are properly sized (as discussed in the paper) and that the op amps are dynamically matched. The second stage of the DOAM instrumentation amplifier presented in this paper may also be used as a DOAM differential amplifier.
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