INA117SM datasheet(9/17 Pages) TI1

Part #	INA117SM

Description	High Common-Mode Voltage DIFFERENCE AMPLIFIER

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INA117

SBOS154A

9

In all cases, the sense resistor imbalances the input resistor

matching of the INA117, degrading its CMR. Also, the input

impedance of the INA117 loads RS, causing gain error in the

voltage-to-current conversion. Both of these errors can be

easily corrected.

The CMR error can be corrected with the addition of a

compensation resistor, RC, equal in value to RS as shown in

Figures 4, 5, and 6. If RS is less than 20Ω, the degradation

in CMR is negligible and RC can be omitted. If RS is larger

than approximately 2k

Ω, trimming R

C may be required to

achieve greater than 86dB CMR. This is because the actual

INA117 input impedances have 1% typical mismatch.

If RS is more than approximately 100Ω, the gain error will

be greater than the 0.02% specification of the INA117. This

gain error can be corrected by slightly increasing the value

of RS. The corrected value, RS', can be calculated by:

R

Rk

kR

S

'

–

=

•Ω

Ω

380

Example: For a 1V/mA transfer function, the nominal,

uncorrected value for RS would be 1kΩ. A slightly larger

value, RS' = 1002.6Ω, compensates for the gain error due to

loading.

The 380k

Ω term in the equation for R

S' has a tolerance of

±25%, so sense resistors above approximately 400Ω may

require trimming to achieve gain accuracy better than 0.02%.

Of course, if a buffer amplifier is added as shown in Figure

7, both inputs see a low source impedance, and the sense

resistor is not loaded. As a result, there is no gain error or

CMR degradation. The buffer amplifier can operate as a

unity gain buffer or as an amplifier with non-inverting gain.

Gain added ahead of the INA117 improves both CMR and

signal-to-noise. Added gain also allows a lower voltage drop

across the sense resistor. The OPA1013 is a good choice for

the buffer amplifier since both its input and output can swing

close to its negative power supply.

FIGURE 7. Current Sensing with Input Buffer.

VX

V1

–21V to +10V

+15V

–5V to –36V

Ground

–20V to –51V

–15V

380k

Ω

380k

Ω

380k

Ω

21.1k

Ω

20k

Ω

47

2

3

81

5

V

O = I • RS • (1 +

)

6

I

–15V

+15V

R

2

R

1

R

2*

1/2

OPA1013

V

1

R *

1

R

S

–V

X

380k

Ω

380k

Ω

380k

Ω

21.1k

Ω

20k

Ω

47

2

3

81

5

V

O = I • RS

6

I

V–

V+

1/2

OPA1013

R

S

0.1µF

IN4702

MPS-A42

180k

Ω

0.01µF

V

Z

or

–V

X

–V

X

Op amp power can be derived with voltage-

dropping zener diode if –V

X power is relatively

constant.

|V

X| = (5V to 36V) + VZ

e.g., If V

Z is 50V then VX = –55V to –86V.

Regulated power for op amp allows –V

X

power to vary over wide range.

V

X = –30V to –200V

*Or connect as buffer (R

2 = 0, omit R1).

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