ICs & Semiconductors
Three op-amp instrumentation amplifier Figure 4 shows the three-op-amp instrumentation amplifier (3-op-amp INA), that amplifies small differential voltages and rejects large common-mode voltages. Its first stage is a pair of high-input impedance buffers (A1, A2) and resistors (RF and RG) that avoid both the input resistive loading effect and the unbalanced input impedances issue. In addition, RF and RG increase the buffer pairs’ difference- mode voltage gains (GDM) to 1 + 2RF/RG while keeping their common-mode voltage gains (GCM) equal to 1. This significantly improves the 3-op-amp INA’s CMRR (CMRR3INA), as CMRR = 20 log (GDM/GCM). Another benefit is that the overall gain of the 3-op-amp INA can be modified by adjusting only the resistance of RG without having to adjust the resistors of R1, R1*, R2and R2*.
The second stage is implemented by a
difference amplifier (A3) which amplifies the difference-mode voltage and rejects the common-mode voltage. In a practical application, the R2/R1 ratio is usually set to 1. CMRR3INA is primarily determined by the difference-mode voltage gain of the first stage and net matching tolerance of R2/R1 and R2*/R1*. The tolerance of resistors RF and RG do not affect CMRR3INA.
For the 3-op-amp INA, there is a common issue that can be easily overlooked, reduced common-mode input voltage range (VCM). Referring to Figure 4, the input voltages (V1, V2) can be represented by common-mode input voltage (VCM) and difference-mode input
Two-op-amp instrumentation amplifier Compared to the 3-op-amp INA, the 2-op- amp INA in Figure 5 provides savings in cost and power consumption. Its input impedances are also very high, which avoids resistive loading effects and the unbalanced input impedances issue. Its common-mode rejection ratio (CMRR2INA) is primarily determined by the overall gain and the net matching tolerance of R2/R1 and R2*/R1*.
As shown in Figure 5, the input voltages (V1, V2) can be represented by common- mode input voltage (VCM) and difference- mode input voltage (VDM). That is, V1 = VCM – VDM/2, and V2 = VCM + VDM/2.
voltage (VDM). That is, V1 = VCM + VDM/2 and V2 = VCM + VDM/2. The amplifiers (A1, A2) provide a
difference-mode voltage gain (GDM), which is equal to the overall gain (G), and a common-mode gain (GCM) equal to 1. VOUT1, VOUT2 and VOUT must remain within the allowed output voltage range between VOL and VOH. The 3-op-amp INA configuration also places specific limits on VDM and VCM and, specifically, its VCM range will be significantly reduced when it operates in a high gain configuration. The 3-op-amp INA offers high common-
mode rejection ratio (CMRR3INA), freedom from resistive loading effects, balanced input impedances and the ability to adjust the overall gain without needing to change more than one resistor value. Conversely, its VCM range is reduced, and the increased number of op amps increases power and cost.
Figure 5. Two op amp instrumentation amplifier
Once again, VOUT and VOUT1 must stay the allowed output voltage range between VOL and VOH, and this configuration also places limits on VDM and VCM values. Unlike the 3-op-amp INA, the VCM range of the 2-op-amp INA will be significantly reduced when it operates in a low-gain configuration. Moreover, the circuit’s asymmetry in its common-mode signal path causes a phase delay between VOUT1 and V1, degrading the AC CMRR performance. Referring to Figure 5, the input signal V1 must pass through amplifier A1 before it can be subtracted from V2 by amplifier A2. Thus, the VOUT1 is slightly delayed and phase shifted with respect to V2. This is a significant limitation. Finally by adding the resistor RG between two inverting inputs, the overall gain of the 2-op-amp INA can be easily set by adjusting only RG. The R2/R1 ratio is usually chosen for the desired minimum
gain. Another benefit of adding the resistor RG is that the large resistor value usage of R2 and R2* can be avoided in very high- gain configurations. As with the other configurations, detail calculations yield the requirements for VDM and VCM, for the of 2-op-amp INA with additional RG. This final circuit arrangement offers high DC common-mode rejection (CMRR2INA), absence of resistive loading effect, balanced input impedances and reduced cost and power consumption, compared to the 3-op-amp INA. Its disadvantages include reduced VCM range, poor AC CMRR2INA, due to the circuit’s asymmetry, and inability to operate at unity gain.
Microchip |
www.microchip.com Darren Wenn is a Senior Field Applications Engineer at Microchip Technology
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Components in Electronics September 2011 29
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