Datasheet OP220 (Analog Devices) - 8

OP220 R0 50mV 2
␮
s GAIN 100 ADJ 90 R1 R2 V1 A1 R4 VCM – 1/2 VD – 10 1/2 R3 0% OP220 VD 20mV A2 V + O VCM + 1/2 VD 1/2 OP220 INPUT OUTPUT
4 È 1 Ê 2 3ˆ 2 3˘ 4 Ê 3 2ˆ
OP220
R R R R R R R R V = Í1+ + V V O ˙ + - R3 2 ËÁ R1 R4 ¯˜ + + R D 0 R3 ËÁ R4 R CM Î ˚ 1¯˜
25k
⍀
100pF
Ê R1ˆ If R1 = R2 = R3 = R4, V then = 2 1+ V O ËÁ R D 0 ¯˜ Figure 2. Small-Signal Transient Response Figure 4. Two Op Amp Instrumentation Amplifier Configuration
2V 200
␮
s
The input voltages are represented as a common-mode input
100
VCM plus a differential input VD. The ratio R3/R4 is made equal
90
to the ratio R2/R, to reject the common-mode input VCM. The differential signal VD is then amplified according to: R4 Ê R3 R2 R3ˆ R3 R2 V = 1 + + + V , where O R3 ËÁ R4 R D ¯˜ = R4 R1
10
O
0% 5V
Note that gain can be independently varied by adjusting RO. From considerations of dynamic range, resistor tempco match- ing, and matching of amplifier response, it is generally best to make RX, R2, R3, and R4 approximately equal. Designating
INPUT
R1, R2, R3, and R4 as RN allows the output equation to be
OUTPUT
further simplified:
OP220
Ê R ˆ
R C
N
L L
V = 2 1+ V , where R R1 R2 R3 R4 O D N
25k
⍀
100pF 40k
⍀ ËÁ R ¯˜ = = = = O
10k
⍀ Dynamic range is limited by A1 as well as A2; the output of A1 is: Figure 3. Large-Signal Transient Response Ê R ˆ V N 1 = - 1 + V 2 V ËÁ R D CM ¯˜ + O
INSTRUMENTATION AMPLIFIER APPLICATIONS OF THE OP220
If the instrumentation amplifier were designed for a gain of 10
OBSOLETE Two Op Amp Configuration
and maximum VD of ±1 V, then RN/RO would need to be four The excellent input characteristics of the OP220 make it ideal for and VO would be a maximum of ±10 V. Amplifier A1 would use in instrumentation amplifier configurations where low-level have a maximum output of ± 5 V plus 2 VCM, thus a limit of differential signals are to be amplified. The low-noise, low input ±10 V on the output of A1 would imply a limit of ±2.5 V on VCM. offsets, low drift, and high gain combined with excellent CMRR A nominal value of 100 kW for RN is suitable for most applica- provide the characteristics needed for high-performance instru- tions. A range of 200 W to 25 kW for RO will then provide a gain mentation amplifiers. In addition, the power supply current range of 10 to 1,000. The current through RO is VD/RO, so the drain is very low. amplifiers must supply ± 10 mV/200 W when the gain is at the The circuit of Figure 4 is recommended for applications where maximum value of 1,000 and VD is at ± 10 mV. the common-mode input range is relatively low and differential Rejecting common-mode inputs is most important in accurately gain will be in the range of 10 to 1,000. This two op amp instrumen- amplifying low-level differential signals. Two factors determine tation amplifier features independent adjustment of common-mode the CMR of this instrumentation amplifier configuration (assuming rejection and differential gain. Input impedance is very high since infinite gain): both inputs are applied to noninverting op amp inputs. 1. CMRR of the op amps 2. Matching of the resistor network (R3/R4 = R2/R1) –8– REV. A 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