Datasheet

AD8628/AD8629/AD8630
Rev. I | Page 18 of 24
PRECISION CURRENT SHUNT SENSOR OUTPUT AMPLIFIER FOR HIGH PRECISION DACS
A precision current shunt sensor benefits from the unique
attributes of auto-zero amplifiers when used in a differencing
configuration, as shown in Figure 63. Current shunt sensors are
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.
The AD8628/AD8629/AD8360 are used as output amplifiers for
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).
R
S
0.1Ω
SUPPLY
I
R
L
100Ω100kΩ
5V
100Ω100kΩ
e = 1000 R
S
I
100mV/mA
AD8628
02735-060
C
C
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 the AD8628/
AD8629/AD8630 minimizes gain errors. The wide bandwidth
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 the AD5541 is 1 µs. The combined settling time is
approximately 1.4 µs, as can be derived from the following
equation:
()
(
)
(
)
2
2
AD8628
SSS
tDACtTOTALt +=
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.
02735-061
AD5541/AD5542
AD8628
DGND
*AD5542 ONLY
V
DD
REF(REFF*) REFS*
V
OUT
SCLK
DIN
CS
AGND
5
V
2.5
V
UNIPOLAR
OUTPUT
LDAC*
0.1µF
10µF
0.1µF
SERIAL
INTERFACE
Figure 64. AD8628 Used as an Output Amplifier