AD620
Precision V-I Converter
The AD620, along with another op amp and two resistors,
makes a precision current source (Figure 42). The op amp
buffers the reference terminal to maintain good CMR. The
output voltage, V
X
, of the AD620 appears across R1, which
converts it to a current. This current, less only the input bias
current of the op amp, then flows out to the load.
+V
S
V
IN+
7
+ V
X
–
INPUT AND OUTPUT OFFSET VOLTAGE
The low errors of the AD620 are attributed to two sources,
input and output errors. The output error is divided by
G
when
referred to the input. In practice, the input errors dominate at
high gains, and the output errors dominate at low gains. The
total
V
OS
for a given gain is calculated as
Total Error RTI = input error + (output error/G)
Total Error RTO = (input error × G) + output error
3
8
R
G
1
REFERENCE TERMINAL
The reference terminal potential defines the zero output voltage
and is especially useful when the load does not share a precise
ground with the rest of the system. It provides a direct means of
injecting a precise offset to the output, with an allowable range
of 2 V within the supply voltages. Parasitic resistance should be
kept to a minimum for optimum CMR.
00775-0-044
AD620
5
4
–V
S
V
x
R1
[(V
IN+
) – (V
IN–
)] G
R1
2
6
R1
V
IN–
I
L
AD705
I
L
=
=
LOAD
INPUT PROTECTION
The AD620 features 400 Ω of series thin film resistance at its
inputs and will safely withstand input overloads of up to ±15 V
or ±60 mA for several hours. This is true for all gains and power
on and off, which is particularly important since the signal
source and amplifier may be powered separately. For longer
time periods, the current should not exceed 6 mA
(I
IN
≤ V
IN
/400 Ω). For input overloads beyond the supplies,
clamping the inputs to the supplies (using a low leakage diode
such as an FD333) will reduce the required resistance, yielding
lower noise.
Figure 42. Precision Voltage-to-Current Converter (Operates on 1.8 mA, ±3 V)
GAIN SELECTION
The AD620’s gain is resistor-programmed by R
G
, or more
precisely, by whatever impedance appears between Pins 1 and 8.
The AD620 is designed to offer accurate gains using 0.1% to 1%
resistors. Table 4 shows required values of R
G
for various gains.
Note that for G = 1, the R
G
pins are unconnected (R
G
= ∞). For
any arbitrary gain, R
G
can be calculated by using the formula:
R
G
=
49.4
k
Ω
G
−
1
RF INTERFERENCE
All instrumentation amplifiers rectify small out of band signals.
The disturbance may appear as a small dc voltage offset. High
frequency signals can be filtered with a low pass R-C network
placed at the input of the instrumentation amplifier. Figure 43
demonstrates such a configuration. The filter limits the input
signal according to the following relationship:
FilterFreq
DIFF
=
FilterFreq
CM
=
1
2
π
R
(2
C
D
+
C
C
)
To minimize gain error, avoid high parasitic resistance in series
with R
G
; to minimize gain drift, R
G
should have a low TC—less
than 10 ppm/°C—for the best performance.
Table 4. Required Values of Gain Resistors
1% Std Table
Value of R
G
(Ω)
49.9 k
12.4 k
5.49 k
2.61 k
1.00 k
499
249
100
49.9
Calculated
Gain
1.990
4.984
9.998
19.93
50.40
100.0
199.4
495.0
991.0
0.1% Std Table
Value of R
G
(Ω )
49.3 k
12.4 k
5.49 k
2.61 k
1.01 k
499
249
98.8
49.3
Calculated
Gain
2.002
4.984
9.998
19.93
49.91
100.0
199.4
501.0
1,003.0
1
2
π
RC
C
where
C
D
≥10C
C.
C
D
affects the difference signal.
C
C
affects the common-mode
signal. Any mismatch in
R
×
C
C
will degrade the AD620’s
CMRR. To avoid inadvertently reducing CMRR-bandwidth
performance, make sure that
C
C
is at least one magnitude
smaller than
C
D
.
The effect of mismatched
C
C
s
is reduced with a
larger
C
D
:C
C
ratio.
Rev. G | Page 16 of 20
AD [ ANALOG DEVICES ]
