AD8638 Analog Devices, AD8638 Datasheet - Page 15

• Manufacturer Part Number: AD8638
• Description: 16 V Auto-Zero, Rail-to-Rail Output Operational Amplifier
• Manufacturer: Analog Devices

Specifications of AD8638

• -3db Bandwidth: 1.5MHz
• Slew Rate: 2V/µs
• Vos: 3µV
• Ib: 1pA
• # Opamps Per Pkg: 1
• Input Noise (nv/rthz): 59nV/rtHz
• Vcc-vee: 5V to 16V
• Isy Per Amplifier: 1.5mA
• Packages: SOIC,SOT-23

INFRARED SENSORS
Infrared (IR) sensors, particularly thermopiles, are increasingly
used in temperature measurement for applications as wide
ranging as automotive climate control, human ear thermometers,
home insulation analysis, and automotive repair diagnostics.
The relatively small output signal of the sensor demands high
gain with very low offset voltage and drift to avoid dc errors.
If interstage ac coupling is used, as shown in Figure 52, low
offset and drift prevent the output of the input amplifier from
drifting close to saturation. The low input bias currents generate
minimal errors from the output impedance of the sensor.
Similar to pressure sensors, the very low amplifier drift with
time and temperature eliminates additional errors once the
system is calibrated at room temperature. The low 1/f noise
improves SNR for dc measurements taken over periods often
exceeding one-fifth of a second.
Figure 52 shows a circuit that can amplify ac signals from
100 μV to 300 μV up to the 1 V to 3 V levels, with a gain of
10,000 for accurate analog-to-digital conversions.
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 53. 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.
SUPPLY
e = 1000 R
100mV/mA
DETECTOR
100µV TO 300µV
Figure 52. AD8639 Used as a Preamplifier for Thermopile
IR
S
I =
100Ω
AD8638
Figure 53. Low-Side Current Sensing
5V TO 16V
100kΩ
100kΩ
5V TO 16V
100kΩ
C
C
1/2 AD8639
f
C
≈ 1.6Hz
10µF
100Ω
100Ω
10kΩ
VOLTAGE
10kΩ
TO BIAS
I
0.1Ω
R
5V TO 16V
S
100kΩ
1/2 AD8639
R
L
Rev. F | Page 15 of 20
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 may 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 to prevent injected bias current
from becoming a significant percentage of the measured current.
High open-loop gain, CMRR, and PSRR help to maintain the
overall circuit accuracy. With the extremely high CMRR of the
AD8638/AD8639, the CMRR is limited by the resistor ratio
matching. 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
The AD8638/AD8639 can be 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 operating 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 offset error when multiplied
by the DAC output impedance (approximately 6 kΩ).
Rail-to-rail output provides full-scale output with very little
error. Output impedance of the DAC is constant and code-
independent, but the high input impedance of the AD8638/
AD8639 minimizes gain errors. The wide bandwidth of the
amplifier also serves well in this case. The amplifier, with a
settling time of 4 μs, adds another time constant to the system,
increasing the settling time of the output. For example, see
Figure 54. The settling time of the AD5541 is 1 μs. The
combined settling time is approximately 4.1 μs, as can be
derived from the following equation:
INTERFACE
SERIAL
*AD5542 ONLY
t
S
(
TOTAL
0.1µF
CS
DIN
SCLK
LDAC*
Figure 54. AD8638 Used as an Output Amplifier
V
5V
DD
)
=
DGND
REF(REFF*) REFS*
AD5541/AD5542
(
t
S
DAC
0.1µF
)
2
AGND
+
(
t
S
2.5V
AD
6
8638
V
ADR421
OUT
4
AD8638/AD8639
)
2
AD8638
5V TO 16V
2
0.1µF
UNIPOLAR
OUTPUT
5V TO 16V