AD8551ARM Analog Devices, AD8551ARM Datasheet - Page 15

• Manufacturer Part Number: AD8551ARM
• Description: Zero-Drift/ Single-Supply/ Rail-to-Rail Input/Output Operational Amplifiers
• Manufacturer: Analog Devices

REV. 0
The optimum value for the resistor and capacitor is a function of
the load capacitance and is best determined empirically since
actual C
stantially from the nominal capacitive load. Table I shows some
snubber network values that can be used as starting points.
Table I. Snubber Network Values for Driving Capacitive Loads
C
1 nF
4.7 nF
10 nF
Power-Up Behavior
On power-up, the AD855x will settle to a valid output within 5 s.
Figure 56a shows an oscilloscope photo of the output of the ampli-
fier along with the power supply voltage, and Figure 56b shows
the test circuit. With the amplifier configured for unity gain, the
device takes approximately 5 s to settle to its final output voltage.
This turn-on response time is much faster than most other auto-
correction amplifiers, which can take hundreds of microseconds or
longer for their output to settle.
LOAD
Figure 55. Overshoot and Ringing are Substantially
Reduced Using a Snubber Network
Figure 56a. AD855x Output Behavior on Power-Up
SNUBBER
SNUBBER
WITHOUT
LOAD
V
OUT
0V
V+
0V
WITH
will include stray capacitances and may differ sub-
BOTTOM TRACE = 2V/DIV
TOP TRACE = 1V/DIV
R
200
60
20
5 s
X
10 s
V
C
SY
LOAD
= +5V
= 4.7nF
C
1 nF
0.47 F
10 F
X
1V
100mV
–15–
APPLICATIONS
A +5 V Precision Strain-Gage Circuit
The extremely low offset voltage of the AD8552 makes it an
ideal amplifier for any application requiring accuracy with high
gains, such as a weigh scale or strain-gage. Figure 57 shows a
configuration for a single supply, precision strain-gage measure-
ment system.
A REF192 provides a +2.5 V precision reference voltage for A2.
The A2 amplifier boosts this voltage to provide a +4.0 V reference
for the top of the strain-gage resistor bridge. Q1 provides the cur-
rent drive for the 350
output of the bridge with the full-scale output voltage equal to:
Where R
in Figure 57, the output voltage will linearly vary from 0 V with
no strain to +4.0 V under full strain.
+3 V Instrumentation Amplifier
The high common-mode rejection, high open-loop gain, and
operation down to +3 V of supply voltage makes the AD855x
an excellent choice of op amp for discrete single supply instru-
mentation amplifiers. The common-mode rejection ratio of
the AD855x is greater than 120 dB, but the CMRR of the sys-
tem is also a function of the external resistor tolerances. The
gain of the difference amplifier shown in Figure 58 is given as:
Figure 56b. AD855x Test Circuit for Turn-On Time
Figure 57. A +5 V Precision Strain-Gage Amplifier
EQUIVALENT
100k
100k
LOAD
350
CELL
V
B
2N2222
OUT
is the resistance of the load cell. Using the values given
NOTE: USE 0.1% TOLERANCE RESISTORS.
+4.0V
OR
Q1
V
1
R
AD8551/AD8552/AD8554
FULL-SCALE
3
2
bridge network. A1 is used to amplify the
1k
AD8552-B
R
40mV
4
12.0k
R
R
4
R
1
B
AD855x
1
R
+5V
17.4k
2
17.4k
A2
R
R
R
R
1
1
2
3
A1
+2.5V
AD8552-A
100
V
100
R
R
2
20k
2
4
V
6
SY
R
R
V
= 0V TO +5V
REF192
2
1
OUT
4
V
0V TO +4.0V
2
OUT
3
(18)
(17)