AD621

REV. B

–13–

Precision V-I Converter

The AD621 along with another op amp and two resistors make

a precision current source (Figure 9). The op amp buffers the

reference terminal to maintain good CMR. The output voltage

rent. This current less only the input bias current of the op amp

then flows out to the load.

AD621

R1

AD705

LOAD

R1

R1

=

Figure 9. Precision Voltage to Current Converter

(Operates on 1.8 mA,

±3 V)

INPUT AND OUTPUT OFFSET VOLTAGE

The AD621 is fully specified for total input errors at gains of 10

and 100. That is, effects of all error sources within the AD621

are properly included in the guaranteed input error specs, elimi-

nating the need for separate error calculation.

Total Error RTI = Input Error + (Output Error/G)

Total Error RTO = (Input Error

× G) + Output Error

REFERENCE TERMINAL

Although usually grounded, the reference terminal may be used

to offset the output of the AD621. This is useful when the load

is “floating” or does not share a ground with the rest of the system.

It also provides a direct means of injecting a precise offset.

Another benefit of having a reference terminal is that it can be

quite effective in eliminating ground loops and noise in a circuit

or system.

AD621

GAIN = 10 OR 100

Figure 10. Input Overload Protection

INPUT OVERLOAD CONSIDERATIONS

Failure of a transducer, faults on input lines, or power supply

sequencing can subject the inputs of an instrumentation ampli-

fier to voltages well beyond their linear range, or even the supply

voltage, so it is essential that the amplifier handle these over-

loads without being damaged.

The AD621 will safely withstand continuous input overloads of

±3.0 volts (±6.0 mA). This is true for gains of 10 and 100, with

power on or off.

The inputs of the AD621 are protected by high current capacity

dielectrically isolated 400

Ω thin-film resistors R3 and R4 (Fig-

ure 3) and by diodes which protect the input transistors Q1 and

Q2 from reverse breakdown. If reverse breakdown occurred, there

would be a permanent increase in the amplifier’s input current.

The input overload capability of the AD621 can be easily increased

while only slightly degrading the noise, common-mode rejection

and offset drift of the device by adding external resistors in series

with the amplifier’s inputs as shown in Figure 10.

Table II summarizes the overload voltages and total input

noise for a range of range of r values. Note that a 2 k

Ω resis-

tor in series with each input will protect the AD621 from a

±15 volt continuous overload, while only increasing input noise

to 13 nV

√Hz—about the same level as would be expected from

a typical unprotected 3 op amp in amp.

Total Input Noise

Maximum Continuous

Value of

in nV

√Hz @ 1 kHz

G = 100

In Volts

014

9

3

499

Ω

14

10

6

1.00 k

Ω

14

11

9

2.00 k

Ω

15

13

15

3.01 k

Ω*

16

14

21

4.99 k

Ω*

17

16

33

*1/4 watt, 1% metal-film resistor. All others are 1/8 watt, 1% RN55

or equivalent.