AD624
10k
1%
INPUT
20V p-p
+V
S
1k
10T
10k
1%
V
OUT
100k
1%
RG
1
G = 100
G = 200
1k
0.1%
500
0.1%
200
0.1%
G = 500
RG
2
AD624
–V
S
Figure 25. Settling Time Test Circuit
THEORY OF OPERATION
The AD624 is a monolithic instrumentation amplifier based on
a modification of the classic three-op-amp instrumentation
amplifier. Monolithic construction and laser-wafer-trimming
allow the tight matching and tracking of circuit components and
the high level of performance that this circuit architecture is ca-
pable of.
A preamp section (Q1–Q4) develops the programmed gain by
the use of feedback concepts. Feedback from the outputs of A1
and A2 forces the collector currents of Q1–Q4 to be constant
thereby impressing the input voltage across R
G
.
The gain is set by choosing the value of R
G
from the equation,
40
k
+ 1. The value of R
G
also sets the transconduct-
Gain =
R
G
ance of the input preamp stage increasing it asymptotically to
the transconductance of the input transistors as R
G
is reduced
for larger gains. This has three important advantages. First, this
approach allows the circuit to achieve a very high open loop gain
of 3
×
10
8
at a programmed gain of 1000 thus reducing gain
related errors to a negligible 3 ppm. Second, the gain bandwidth
product which is determined by C3 or C4 and the input trans-
conductance, reaches 25 MHz. Third, the input voltage noise
reduces to a value determined by the collector current of the
input transistors for an RTI noise of 4 nV/√Hz at G
≥
500.
+V
S
+V
S
16.2k
The AD524 should be considered in applications that require
protection from severe input overload. If this is not possible,
external protection resistors can be put in series with the inputs
of the AD624 to augment the internal (50
Ω)
protection resis-
tors. This will most seriously degrade the noise performance.
For this reason the value of these resistors should be chosen to
be as low as possible and still provide 10 mA of current limiting
under maximum continuous overload conditions. In selecting
the value of these resistors, the internal gain setting resistor and
the 1.2 volt drop need to be considered. For example, to pro-
tect the device from a continuous differential overload of 20 V
at a gain of 100, 1.9 kΩ of resistance is required. The internal
gain resistor is 404
Ω;
the internal protect resistor is 100
Ω.
There is a 1.2 V drop across D1 or D2 and the base-emitter
junction of either Q1 and Q3 or Q2 and Q4 as shown in Figure
27, 1400
Ω
of external resistance would be required (700
Ω
in
series with each input). The RTI noise in this case would be
4
KTR
ext
+(4
nV
/
Hz
)
2
=
6.2
nV
/
Hz
+V
S
I1
50 A
A1
VB
I2
50 A
R52
10k
SENSE
C3
A2
C4
R53
10k
A3
RG
2
500
124
4445
200
225.3
100
–V
S
R56
20k
Q2,
Q4
R54
10k
V
O
50
100
200
500
RG
2
1 F
G500
–V
S
Q1, Q3
R57
20k
RG
1
–IN
1 F
1/2
AD712
16.2k
–V
S
1.62M
1 F
1.82k
AD624
1/2
AD712
9.09k
G1, 100, 200
1k
100
13
50 A
80.2
I4
50 A
50
R55
10k
REF
+IN
Figure 26. Noise Test Circuit
Figure 27. Simplified Circuit of Amplifier; Gain Is Defined
as (R56 + R57)/(R
G
) + 1. For a Gain of 1, R
G
Is an Open
Circuit.
INPUT OFFSET AND OUTPUT OFFSET
Under input overload conditions the user will see R
G
+ 100
Ω
and two diode drops (~1.2 V) between the plus and minus
inputs, in either direction. If safe overload current under all
conditions is assumed to be 10 mA, the maximum overload
voltage is ~
±
2.5 V. While the AD624 can withstand this con-
tinuously, momentary overloads of
±
10 V will not harm the
device. On the other hand the inputs should never exceed the
supply voltage.
REV. C
–7–
INPUT CONSIDERATIONS
Voltage offset specifications are often considered a figure of
merit for instrumentation amplifiers. While initial offset may
be adjusted to zero, shifts in offset voltage due to temperature
variations will cause errors. Intelligent systems can often correct
for this factor with an autozero cycle, but there are many small-
signal high-gain applications that don’t have this capability.
Voltage offset and offset drift each have two components; input
and output. Input offset is that component of offset that is
AD [ ANALOG DEVICES ]
