AD8629 Analog Devices, AD8629 Datasheet - Page 18

• Manufacturer Part Number: AD8629
• Description: Zero Drift, Single-Supply, R/R, Input/Output Operational Amplifier
• Manufacturer: Analog Devices

Specifications of AD8629

• -3db Bandwidth: 2.5MHz
• Slew Rate: 1V/µs
• Vos: 1µV
• Ib: 30pA
• # Opamps Per Pkg: 2
• Input Noise (nv/rthz): 22nV/rtHz
• Vcc-vee: 2.7V to 6V
• Isy Per Amplifier: 1.1mA
• Packages: SOIC,SOP

AD8628/AD8629/AD8630
PRECISION CURRENT SHUNT SENSOR
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.
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.
SUPPLY
e = 1000 R
100mV/mA
S
I
Figure 63. Low-Side Current Sensing
AD8628
100kΩ
100kΩ
5V
C
C
100Ω
100Ω
I
0.1Ω
R
S
R
L
Rev. I | Page 18 of 24
OUTPUT AMPLIFIER FOR HIGH PRECISION DACS
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).
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:
INTERFACE
SERIAL
*AD5542 ONLY
t
S
(
TOTAL
0.1µF
CS
DIN
SCLK
LDAC*
Figure 64. AD8628 Used as an Output Amplifier
V
5V
)
DD
=
DGND
REF(REFF*) REFS*
(
AD5541/AD5542
t
S
DAC
2.5V
0.1µF
)
2
+
10µF
AGND
(
t
S
AD8628
V
OUT
)
2
AD8628
UNIPOLAR
OUTPUT