AD620

REV. E

–13–

Precision V-I Converter

The AD620, along with another op amp and two resistors, makes

a precision current source (Figure 37). The op amp buffers the

reference terminal to maintain good CMR. The output voltage

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 37. Precision Voltage-to-Current Converter

(Operates on 1.8 mA,

±3 V)

GAIN SELECTION

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

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

49.4 k

Ω

G

− 1

To minimize gain error, avoid high parasitic resistance in series

than 10 ppm/

°C—for the best performance.

Table II. Required Values of Gain Resistors

1% Std Table

Calculated

0.1% Std Table

Calculated

Gain

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

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

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 can rectify out of band signals,

and when amplifying small signals, these rectified voltages act as

small dc offset errors. The AD620 allows direct access to the

input transistor bases and emitters enabling the user to apply

some first order filtering to unwanted RF signals (Figure 38),

where RC

1/(2

πf) and where f ≥ the bandwidth of the

AD620; C

≤ 150 pF. Matching the extraneous capacitance at

Pins 1 and 8 and Pins 2 and 3 helps to maintain high CMR.

–IN

1

2

3

4

5

6

7

8

R

R

+IN

C

C

Figure 38. Circuit to Attenuate RF Interference