AD620 查看數據表（PDF） - Analog Devices

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AD620

Low Cost Low Power Instrumentation Amplifier

Analog Devices 

AD620

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, VX, 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.

+VS

VIN+

RG

VIN–

3

7

8

AD620

6

1

5

2

4

+ VX –

R1

–VS

I L=

Vx = [(VIN+) – (VIN–)] G

R1

R1

AD705

IL

LOAD

Figure 40. Precision Voltage-to-Current Converter (Operates on 1.8 mA, ±3 V)

GAIN SELECTION

The AD620 gain is resistor-programmed by RG, 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 RG for various gains.

Note that for G = 1, the RG pins are unconnected (RG = ∞). For

any arbitrary gain, RG can be calculated by using the formula:

49.4 kΩ

RG = G −1

To minimize gain error, avoid high parasitic resistance in series

with RG; to minimize gain drift, RG should have a low TC—less

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

Table 5. Required Values of Gain Resistors

1% Std Table Calculated 0.1% Std Table

Value of RG(Ω) Gain

Value of RG(Ω )

49.9 k

1.990

49.3 k

12.4 k

4.984

12.4 k

5.49 k

9.998

5.49 k

2.61 k

19.93

2.61 k

1.00 k

50.40

1.01 k

499

100.0

499

249

199.4

249

100

495.0

98.8

49.9

991.0

49.3

Calculated

Gain

2.002

4.984

9.998

19.93

49.91

100.0

199.4

501.0

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 VOS 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.

+SUPPLY

R

+IN

AD620

VOUT

R

REF

–IN

–SUPPLY

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

Rev. H | Page 15 of 20

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