Datasheet

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 R
S
, 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, R
C
, equal in value to R
S
as shown in
Figures 4, 5, and 6. If R
S
is less than 20Ω, the degradation
in CMR is negligible and R
C
can be omitted. If R
S
is larger
than approximately 2kΩ, 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 R
S
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 R
S
. The corrected value, R
S
', can be calculated by:
R
Rk
kR
S
S
S
'
–
=
•Ω
Ω
380
380
Example: For a 1V/mA transfer function, the nominal,
uncorrected value for R
S
would be 1kΩ. A slightly larger
value, R
S
' = 1002.6Ω, compensates for the gain error due to
loading.
The 380kΩ 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.
V
X
V
1
–21V to +10V +15V
–5V to –36V Ground
–20V to –51V –15V
380kΩ 380kΩ
380kΩ
21.1kΩ 20kΩ
47
2
3
815
V
O
= I • R
S
• (1 + )
6
I
–15V +15V
R
2
R
1
R
2
*
1/2
OPA1013
V
1
R *
1
R
S
–V
X
380kΩ 380kΩ
380kΩ
21.1kΩ 20kΩ
47
2
3
815
V
O
= I • R
S
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) + V
Z
e.g., If V
Z
is 50V then V
X
= –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 R
1
).