Datasheet AD621 (Analog Devices) - 10

AD621 +VS
R5 at a gain of 10 or the parallel combination of R5 and R6 at a
7
gain of 100.
I1 20
␮
A VB 20
␮
A I2
This creates a differential gain from the inputs to the A1/A2 outputs given by G = (R1 + R2) / RG + 1. The unity-gain
– – + +
subtracter A3 removes any common-mode signal, yielding a
A1 A2 10k
⍀ single-ended output referred to the REF pin potential.
C1 C2 10k
⍀ The value of RG also determines the transconductance of the
– OUTPUT A3
preamp stage. As RG is reduced for larger gains, the transcon-
+ 6 10k
⍀ ductance increases asymptotically to that of the input transistors.
R1 25k
⍀
R3 R2 25k
⍀
10k
⍀
400
⍀ This has three important advantages: (a) Open-loop gain is
R5 REF –IN Q1 5 5555.6
⍀
Q2 +IN
boosted for increasing programmed gain, thus reducing gain-
2 R4 3 R6 400
⍀ related errors. (b) The gain-bandwidth product (determined by
555.6
⍀
1 8
C1, C2 and the preamp transconductance) increases with pro-
G = 100 G = 100
grammed gain, thus optimizing frequency response. (c) The input voltage noise is reduced to a value of 9 nV/√Hz, deter-
4
mined mainly by the collector current and base resistance of the
–VS
input devices. Figure 3. Simplified Schematic of AD621
Make vs. Buy: A Typical Bridge Application Error Budget
The AD621 offers improved performance over discrete three op
THEORY OF OPERATION
amp IA designs, along with smaller size, fewer components and The AD621 is a monolithic instrumentation amplifier based on 10 times lower supply current. In the typical application, shown a modification of the classic three op amp circuit. Careful layout in Figure 4, a gain of 100 is required to amplify a bridge output of of the chip, with particular attention to thermal symmetry builds 20 mV full scale over the industrial temperature range of –40°C to in tight matching and tracking of critical components, thus +85°C. The error budget table below shows how to calculate preserving the high level of performance inherent in this circuit, the effect various error sources have on circuit accuracy. at a low price. Regardless of the system it is being used in, the AD621 provides On chip gain resistors are pretrimmed for gains of 10 and 100. greater accuracy, and at low power and price. In simple systems, The AD621 is preset to a gain of 10. A single external jumper absolute accuracy and drift errors are by far the most significant (between Pins 1 and 8) is all that is needed to select a gain of contributors to error. In more complex systems with an intelligent 100. Special design techniques assure a low gain TC of 5 ppm/°C processor, an autogain/autozero cycle will remove all absolute max, even at a gain of 100. accuracy and drift errors leaving only the resolution errors of Figure 3 is a simplified schematic of the AD621. The input gain nonlinearity and noise, thus allowing full 14-bit accuracy. transistors Q1 and Q2 provide a single differential-pair bipolar Note that for the discrete circuit, the OP07 specifications for input for high precision, yet offer 10× lower Input Bias Current, input voltage offset and noise have been multiplied by 2. This is thanks to Superβeta processing. Feedback through the Q1-A1-R1 because a three op amp type in amp has two op amps at its inputs, loop and the Q2-A2-R2 loop maintains constant collector cur- both contributing to the overall input error. rent of the input devices Q1 and Q2, thereby impressing the input voltage across the gain-setting resistor, RG, which equals
10V + 10k
⍀*
10k
⍀*
OP07D – + 10k
⍀**
R = 350
⍀
R = 350
⍀
– AD621A 100k
⍀**
OP07D + – REFERENCE 10k
⍀**
R = 350
⍀
R = 350
⍀
– OP07D + 10k
⍀*
10k
⍀*
AD621A MONOLITHIC INSTRUMENTATION AMPLIFIER, G = 100 3 OP AMP, IN AMP, G = 100
*
0.02% RESISTOR MATCH, 3PPM/
ⴗ
C TRACKING PRECISION BRIDGE TRANSDUCER SUPPLY CURRENT = 1.3mA MAX
**
DISCRETE 1% RESISTOR, 100PPM/
ⴗ
C TRACKING SUPPLY CURRENT = 15mA MAX
Figure 4. Make vs. Buy –10– REV. B