AD8551/AD8552/AD8554
10s
V
= 0V TO +5V
SY
100k⍀
100k⍀
WITH
SNUBBER
V
OUT
AD855x
Figure 56b. AD855x Test Circuit for Turn-On Time
APPLICATIONS
WITHOUT
SNUBBER
V
C
= +5V
LOAD
SY
A +5 V Precision Strain-Gage Circuit
100mV
= 4.7nF
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.
Figure 55. Overshoot and Ringing are Substantially
Reduced Using a Snubber Network
The optimum value for the resistor and capacitor is a function of
the load capacitance and is best determined empirically since
actual CLOAD will include stray capacitances and may differ sub-
stantially from the nominal capacitive load. Table I shows some
snubber network values that can be used as starting points.
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 Ω bridge network. A1 is used to amplify the
output of the bridge with the full-scale output voltage equal to:
Table I. Snubber Network Values for Driving Capacitive Loads
2 × R + R
(
)
1
2
(17)
CLOAD
RX
CX
RB
1 nF
4.7 nF
10 nF
200 Ω
60 Ω
20 Ω
1 nF
0.47 µF
10 µF
Where RB is the resistance of the load cell. Using the values given
in Figure 57, the output voltage will linearly vary from 0 V with
no strain to +4.0 V under full strain.
Power-Up Behavior
2
On power-up, the AD855x will settle to a valid output within 5 µs.
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 µs 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.
+5V
+2.5V
3
Q1
2N2222
REF192
1k⍀
6
A2
OR
4
AD8552-B
EQUIVALENT
12.0k⍀
20k⍀
+4.0V
R
R
1
2
17.4k⍀
100⍀
40mV
FULL-SCALE
V
A1
OUT
350⍀
LOAD
CELL
0V TO +4.0V
AD8552-A
R
R
3
4
17.4k⍀
100⍀
V
OUT
0V
NOTE: USE 0.1% TOLERANCE RESISTORS.
Figure 57. A +5 V Precision Strain-Gage Amplifier
+3 V Instrumentation Amplifier
V+
0V
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:
5s
1V
BOTTOM TRACE = 2V/DIV
TOP TRACE = 1V/DIV
Figure 56a. AD855x Output Behavior on Power-Up
R4
R3 + R4
R1
R2
R2
R1
VOUT =V1
1+
−V2
(18)
REV. 0
–15–
ADI [ ADI ]
