AD8628/AD8629
OUTPUT AMPLIFIER FOR HIGH PRECISION DACs
PRECISION CURRENT SHUNTS
The AD8628/AD8629 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 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. 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).
A precision shunt current sensor benefits from the unique
attributes of auto-zero amplifiers when used in a differencing
configuration (Figure 63). Shunt current 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.
Rail-to-rail input and output provide full-scale output with very
little error. Output impedance of the DAC is constant and code-
independent, but the high input impedance of the AD8628/
AD8629 minimizes gain errors. The amplifiers’ wide bandwidth
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:
R
0.1Ω
S
R
SUPPLY
L
I
100kΩ
100Ω
e = 1,000 R
100mV/mA
I
S
C
5V
AD8628
100kΩ
100Ω
2
2
tS
(
TOTAL
)
=
tS DAC
)
+
(
tS AD8628
)
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
without saving power. A typical shunt might be 0.1 Ω. At
measured current values of 1 A, the shunt’s output signal is
hundreds of mV, or even V, 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 all 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.
2.5V
5V
10µF
0.1µF
0.1µF
SERIAL
V
REF(REF*) REFS*
AD5541/AD5542
DD
INTERFACE
CS
DIN
UNIPOLAR
OUTPUT
OUT
SCLK
LDAC*
AD8628
DGND
AGND
*AD5542 ONLY
Figure 64. AD8628 Used as an Output Amplifier
Rev. C | Page 18 of 20
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