Datasheet

The MAX9938 allows two methods of filtering to help
improve performance in the presence of input common-
mode voltage and input differential voltage transients.
Figure 1 shows a differential input filter.
The capacitor CIN between RS+ and RS- along with the
resistor RIN between the sense resistor and RS- helps
filter against input differential voltages and prevents
them from reaching the MAX9938.
The corner frequency of this filter is determined by the
choice of RIN, CIN, and the value of the input resis-
tance at RS- (R
1
). See Table 1 for R
1
values at the dif-
ferent gain options.
The value of RIN should be chosen to minimize its
effect on the input offset voltage due to the bias current
at RS-. RIN x I
BIAS
contributes to the input voltage off-
set. I
BIAS
is typically 0.2μA.
Placing RIN at the RS- input does not affect the gain
error of the device because the gain is given by the
ratio between R
OUT
and R
1
at RS+.
Figure 2 shows the input common-mode filter.
Again, the corner frequency of the filter is determined
by the choice of R
IN
, C
IN
and is affected by R
1
.
In this case R
IN
affects both gain error and input offset
voltage. R
IN
should be smaller than R
1
so that it has
negligible effect on the device gain. If, for example, a fil-
ter with R
IN
= 10Ω and C
IN
= 1μF is built, then depend-
ing upon the gain selection, the gain error is affected by
either 2.5% (G = 25V/V, R
1
= 400Ω) or 5% (G = 50V/V,
R
1
= 200Ω) or 10% (G = 100V/V, R
1
= 100Ω) or 10%
(G = 200V/V, R
1
= 100Ω).
MAX9938
1µA, 4-Bump UCSP/SOT23,
Precision Current-Sense Amplifier
8 _______________________________________________________________________________________
Figure 2. Input Common-Mode Filter
R
SENSE
C
IN
C
IN
R
IN
R
IN
RS-
OUT
GND
RS+
MAX9938
LOAD
R
SENSE
C
IN
R
IN
RS-
OUT
GND
RS+
MAX9938
LOAD
Figure 1. Differential Input Filter