AD622
THEORY OF OPERATION
Make vs. Buy: A Typical Application Error Budget
The AD622 is a monolithic instrumentation amplifier based on
a modification of the classic three op-amp approach. Absolute
value trimming allows the user to program gain
accurately
(to
0.5% at G = 100) with only one resistor. Monolithic construc-
tion and laser wafer trimming allow the tight matching and
tracking of circuit components, thus insuring its performance.
The input transistors Q1 and Q2 provide a single differential-
pair bipolar input for high precision. Feedback through the
Q1-A1-R1 loop and the Q2-A2-R2 loop maintains constant
collector current of the input devices Q1, Q2 thereby impressing
the input voltage across the external gain-setting resistor R
G
.
This creates a differential gain from the inputs to the A1/A2
outputs given by G = (R1 + R2)/R
G
+ 1. The unity-gain sub-
tracter A3 removes any common-mode signal, yielding a
single-ended output referred to the REF pin potential.
The value of R
G
also determines the transconductance of the
preamp stage. As R
G
is reduced for larger gains, the transcon-
ductance increases asymptotically to that of the input transistors.
This has three important advantages: (a) Open-loop gain is
boosted for increasing programmed gain, thus reducing gain-
related errors. (b) The gain-bandwidth product (determined by
C1, C2 and the preamp transconductance) increases with pro-
grammed gain, thus optimizing frequency response. (c) The
input voltage noise is reduced to a value of 12 nV/√Hz, deter-
mined mainly by the collector current and base resistance of the
input devices.
The internal gain resistors, R1 and R2, are trimmed to an abso-
lute value of 25.25 kΩ, allowing the gain to be programmed
accurately with a single external resistor.
The AD622 offers a cost and performance advantages over
discrete “two op-amp” instrumentation amplifier designs along
with smaller size and less components. In a typical application
shown in Figure 14, a gain of 10 is required to receive and am-
plify a 0–20 mA signal from the AD694 current transmitter.
The current is converted to a voltage in a 50
Ω
shunt. In appli-
cations where transmission is over long distances, line imped-
ance can be significant so that differential voltage measurement
is essential. Where there is no connection between the ground
returns of transmitter and receiver, there must be a dc path from
each input to ground, implemented in this case using two 1 kΩ
resistors. The error budget detailed in Table I shows how to
calculate the effect various error sources have on circuit accuracy.
The AD622 provides greater accuracy at lower cost. The higher
cost of the “homebrew” circuit is dominated in this case by the
matched resistor network. One could also realize a “homebrew”
design using cheaper discrete resistors which would be either
trimmed or hand selected to give high common-mode rejection.
This level of common-mode rejection would however degrade
significantly over temperature due to the drift mismatch of the
discrete resistors.
Note that for the homebrew circuit, the LT1013 specification
for noise has been multiplied by
√2.
This is because a “two op-
amp” type instrumentation amplifier has two op amps at its
inputs, both contributing to the overall noise.
1/2
LT1013
R
L2
10
V
IN
1k
0–20mA
50
R
G
5.62k
1k
AD694
0–20mA
TRANSMITTER
AD622
REFERENCE
1k
9k *
1/2
LT1013
R
L2
10
1k
1k *
1k *
9k *
*0.1% RESISTOR MATCH, 50ppm / C TRACKING
0–20 mA Current Loop
with 50
Ω
Shunt Impedance
AD622 Monolithic
Instrumentation Amplifier,
G = 9.986
Figure 14. Make vs. Buy
“Homebrew” In Amp, G = 10
REV. C
–7–
AD [ ANALOG DEVICES ]
