AD8554 datasheet(15/20 Pages) AD

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AD8551/AD8552/AD8554

–15–

REV. 0

10 s

100mV

WITH

SNUBBER

WITHOUT

SNUBBER

VSY = +5V

CLOAD = 4.7nF

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.

Table I. Snubber Network Values for Driving Capacitive Loads

CLOAD

1 nF

200

Ω

1 nF

4.7 nF

Ω

0.47

µF

10 nF

Ω

µF

Power-Up Behavior

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.

5 s

VOUT

V+

BOTTOM TRACE = 2V/DIV

TOP TRACE = 1V/DIV

Figure 56a. AD855x Output Behavior on Power-Up

VSY = 0V TO +5V

100k

AD855x

100k

VOUT

Figure 56b. AD855x Test Circuit for Turn-On Time

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

Ω bridge network. A1 is used to amplify the

output of the bridge with the full-scale output voltage equal to:

×+

()

(17)

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.

VOUT

350

LOAD

CELL

AD8552-A

17.4k

100

0V TO +4.0V

NOTE: USE 0.1% TOLERANCE RESISTORS.

20k

AD8552-B

REF192

12.0k

+5V

+2.5V

+4.0V

40mV

FULL-SCALE

2N2222

EQUIVALENT

17.4k

100

Figure 57. A +5 V Precision Strain-Gage Amplifier

+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:

OUT =

















−









(18)

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