Data Sheet
AD8628/AD8629/AD8630
Rev. J | Page 19 of 24
PRECISION CURRENT SHUNT SENSOR
A precision current shunt sensor benefits from the unique
attributes of auto-zero amplifiers when used in a differencing
used in precision current sources for feedback control systems.
They are also used in a variety of other applications, including
battery fuel gauging, laser diode power measurement and control,
torque feedback controls in electric power steering, and precision
power metering.
RS
0.1
SUPPLY
I
RL
100
100k
5V
100
100k
e = 1000 RS I
100mV/mA
AD8628
02
73
5-
0
60
C
Figure 63. Low-Side Current Sensing
In such applications, it is desirable to use a shunt with very low
resistance to minimize the series voltage drop; this minimizes
wasted power and allows the measurement of high currents
while saving power. A typical shunt might be 0.1 Ω. At measured
current values of 1 A, the output signal of the shunt is hundreds
of millivolts, or even volts, and amplifier error sources are not
critical. However, at low measured current values in the 1 mA
range, the 100 μV output voltage of the shunt demands a very
low offset voltage and drift to maintain absolute accuracy. Low
input bias currents are also needed, so that injected bias current
does not become a significant percentage of the measured current.
High open-loop gain, CMRR, and PSRR help to maintain the
overall circuit accuracy. As long as the rate of change of the
current is not too fast, an auto-zero amplifier can be used with
excellent results.
OUTPUT AMPLIFIER FOR HIGH PRECISION DACS
a 16-bit high precision DAC in a unipolar configuration. In this
case, the selected op amp needs to have a very low offset voltage
(the DAC LSB is 38 μV when operated with a 2.5 V reference)
to eliminate the need for output offset trims. The input bias
current (typically a few tens of picoamperes) must also be very
low because it generates an additional zero code error when
multiplied by the DAC output impedance (approximately 6 kΩ).
Rail-to-rail input and output provide full-scale output with very
little error. The output impedance of the DAC is constant and
code independent, but the high input impedance of th
e AD8628/of the amplifiers also serves well in this case. The amplifiers,
with settling time of 1 μs, add another time constant to the
system, increasing the settling time of the output. The settling
time of th
e AD5541 is 1 μs. The combined settling time is
approximately 1.4 μs, as can be derived from the following
equation:
2
2
AD8628
S
t
DAC
t
TOTAL
t
02
73
5-
0
61
AD5541/AD5542
AD8628
DGND
*AD5542 ONLY
VDD REF(REFF*) REFS*
VOUT
SCLK
DIN
CS
AGND
5V
2.5V
UNIPOLAR
OUTPUT
LDAC*
0.1F
10F
0.1F
SERIAL
INTERFACE