AD620
Rev. H | Page 15 of 20
Precision V-I Converter
The AD620, along with another op amp and two resistors,
makes a precision current source (Figure 40). 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.
AD620
R
G
–V
S
V
IN+
V
IN–
LOAD
R1
I
L
V
x
I =
L
R1
=
IN+
[(V ) – (V )] G
IN–
R1
6
5
+ V –
X
4
2
1
8
3
7
+V
S
AD705
00775-0-044
Figure 40. Precision Voltage-to-Current Converter (Operates on 1.8 mA, ±3 V)
GAIN SELECTION
The AD620 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 5 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:
1
4.49
−
Ω
=
G
k
R
G
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 5. Required Values of Gain Resistors
1% Std Table
Value of R
G
(Ω)
Calculated
Gain
0.1% Std Table
Value of R
G
(Ω )
Calculated
Gain
49.9 k 1.990 49.3 k 2.002
12.4 k 4.984 12.4 k 4.984
5.49 k 9.998 5.49 k 9.998
2.61 k 19.93 2.61 k 19.93
1.00 k 50.40 1.01 k 49.91
499 100.0 499 100.0
249 199.4 249 199.4
100 495.0 98.8 501.0
49.9 991.0 49.3 1,003.0
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
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.
INPUT PROTECTION
The AD620 safely withstands an input current of ±60 mA for
several hours at room temperature. This is true for all gains and
power on and off, which is useful if the signal source and
amplifier are powered separately. For longer time periods, the
input current should not exceed 6 mA.
For input voltages beyond the supplies, a protection resistor
should be placed in series with each input to limit the current to
6 mA. These can be the same resistors as those used in the RFI
filter. High values of resistance can impact the noise and AC
CMRR performance of the system. Low leakage diodes (such as
the BAV199) can be placed at the inputs to reduce the required
protection resistance.
AD620
R
REF
R
+SUPPL
–SUPPLY
V
OUT
+IN
–IN
00775-0-052
Figure 41. Diode Protection for Voltages Beyond Supply
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 42
demonstrates such a configuration. The filter limits the input
