Datasheet OP220 (Analog Devices) - 9

OP220
In this instrumentation amplifier configuration, error due to
THREE OP AMP CONFIGURATION
CMRR effect is directly proportional to the differential CMRR A three op amp instrumentation amplifier configuration using of the op amps. For the OP220A/E, this combined CMRR is a the OP220 and OP777 is recommended for applications requiring minimum of 98 dB. A combined CMRR value of 100 dB and high accuracy over a wide gain range. This circuit provides common-mode input range of ± 2.5 V indicates a peak input- excellent CMR over a wide input range. As with the two op amp referred error of only ± 25 mV. instrumentation amplifier circuits, tight matching of the two op Resistor matching is the other factor affecting CMRR. Defining amps provides a real boost in performance. Ad as the differential gain of the instrumentation amplifier and
R1
assuming that R1, R2, R3 and R4 are approximately equal (R
2R1
N
VO = VD 1 + R0
will be the nominal value), then CMRR will be approximately AD divided by 4DR/RN. CMRR at differential gain of 100 would
R2 R2 A1
be 88 dB with resistor matching of 0.1%. Trimming R1 to make
VCM – 1/2 VD V1 V+ –
the ratio R3/R4 equal to R2/R1 will directly raise the CMRR
1/2 OP777
until it is limited by linearity and resistor stability considerations.
OP220 R0 A3 VO
The high open-loop gain of the OP220 is very important in
R1 VD
achieving high accuracy in the two-op-amp instrumentation
V+
amplifier configuration. Gain error can be approximated by:
R2 A2 V– +
1 A
VCM + 1/2 VD V2 R2
Gain Error = D , < 1
1/2
A 2A A
OP220 V–
1 + D 01 02 A 02 Figure 5. Three Op Amp Instrumentation Amplifier Using where A OP220 and OP777 D is the instrumentation amplifier differential gain and A02 is the open-loop gain of op amp A2. This analysis assumes A simplified schematic is shown in Figure 2. The input stage equal values of R1, R2, R3, and R4. For example, consider an (A1 and A2) serves to amplify the differential input VD without OP220 with A02 of 700 V/mV. If the differential gain AD were amplifying the common-mode voltage VCM. The output stage set to 700, the gain error would be 1/1.001 which is approxi- then rejects the common-mode input. With ideal op amps and mately 0.1%. no resistor matching errors, the outputs of each amplifier will be: Another effect of finite op amp gain is undesired feedthrough of common-mode input. Defining A01 as the open-loop gain of op Ê R 2 1ˆ VD amp A1, then the common-mode error (CME) at the output V 1 = - 1 + V ËÁ R CM ¯˜ + 2 due to this effect will be approximately: O Ê R 2 1ˆ V V D 2 = 1 + V 2A 1 CM CME D = V ËÁ R ¯˜ + 2 O A A CM D 1 + 01 Ê R 2 1ˆ A V =V 2 -V1 = 1+ V 01 O ËÁ R D ¯˜ O For AD/A01, < 1, this simplifies to (2 AD/A01) ⫻ VCM. If the op V = A V O D D amp gain is 700 V/mV, V CM is 2.5 V, and AD is set to 700, then the error at the output due to this effect will be approximately 5 mV. The differential gain AD is 1 + 2R1/RO and the common-mode The OP220 offers a unique combination of excellent dc perfor- input VCM is rejected. mance, wide input range, and low supply current drain that is particularly attractive for instrumentation amplifier design. This three op amp instrumentation amplifier configuration using an OP220 at the input and an OP777 at the output provides excellent
OBSOLETE
performance over a wide gain range with very low power consump- tion. A gain range of 1 to 2,000 is practical and CMR of over 120 dB is readily achievable. REV. A –9– Document Outline FEATURES GENERAL DESCRIPTION PIN CONFIGURATIONS SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS DIE CHARACTERISTICS ORDERING GUIDE WAFER TEST LIMITS Typical Performance Characteristics INSTRUMENTATION AMPLIFIER APPLICATIONS OF THE OP220 Two Op Amp Configuration THREE OP AMP CONFIGURATION OUTLINE DIMENSIONS Revision History