AD8628/AD8629/AD8630
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 ±6-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 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 ± μs, add another time constant to the
system, increasing the settling time of the output. The settling
time of the AD554± is ± μs. The combined settling time is
approximately ±.4 μs, as can be derived from the following
equation:
R
0.1Ω
S
R
SUPPLY
L
I
100kΩ
100Ω
e = 1000 R
100mV/mA
I
S
C
5V
AD8628
100kΩ
100Ω
C
2
2
tS
(TOTAL
)
=
)
+
)
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.± Ω. At measured
current values of ± 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 ± mA
range, the ±00 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.
5V
2.5V
10µF
0.1µF
0.1µF
SERIAL
V
REF(REFF*) REFS*
AD5541/AD5542
DD
INTERFACE
CS
AD8628
DIN
UNIPOLAR
OUTPUT
V
OUT
SCLK
LDAC*
DGND
AGND
*AD5542 ONLY
Figure 64. AD8628 Used as an Output Amplifier
Rev. I | Page 18 of 24
ROCHESTER [ Rochester Electronics ]
