AD620

Rev. G | Page 16 of 20

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

converts it to a current. This current, less only the input bias

current of the op amp, then flows out to the load.

AD620

V

IN–

LOAD

R1

I

L

I =

L

R1

=

IN+

[(V

) – (V

)] G

IN–

R1

6

5

+ V –

X

4

2

1

8

3

7

AD705

Figure 42. Precision Voltage-to-Current Converter (Operates on 1.8 mA, ±3 V)

GAIN SELECTION

precisely, by whatever impedance appears between Pins 1 and 8.

The AD620 is designed to offer accurate gains using 0.1% to 1%

1

4

.

49

−

Ω

=

G

k

R

G

To minimize gain error, avoid high parasitic resistance in series

than 10 ppm/°C—for the best performance.

Table 4. Required Values of Gain Resistors

1% Std Table

Calculated

Gain

0.1% Std Table

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 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 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

clamping the inputs to the supplies (using a low leakage diode

such as an FD333) will reduce the required resistance, yielding

lower noise.

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:

)

2

(

2

1

C

D

DIFF

C

C

R

FilterFreq

+

π

=

C

CM

RC

FilterFreq

π

=

2

1

CMRR. To avoid inadvertently reducing CMRR-bandwidth