AMP03(2010) Просмотр технического описания (PDF) - Aavid Thermalloy, LLC
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AMP03 Datasheet PDF : 16 Pages
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INCREASING INPUT IMPEDANCE
In applications where a high input impedance is needed, low
input bias current op amps can be used to buffer the AD8275.
In Figure 41, an AD8620 is used to provide high input imped-
ance. Input bias current is limited to 10 pA.
+13V
+5V
0.1µF
INVERTING
INPUT
3
2
8
AD8620
1/2
0.1µF
1
50kΩ
2
–IN
7
+VS
10kΩ SENSE
5
OUT
6
VOUT
6
5
NON-
INVERTING
INPUT
AD8620
7
2/2
4
–13V
0.1µF
50kΩ
3
+IN
20kΩ
20kΩ
REF2
8
REF1
1
AD8275 –VS
4
VREF
Figure 41. Adding Op Amp Buffers for High Input Impedance
AC COUPLING
An integrator can be tied to the AD8275 in feedback to create a
high-pass filter as shown in Figure 42. This circuit can be used
to reject dc voltages and offsets. At low frequencies, the impedance
of the capacitor, C, is high. Thus, the gain of the integrator is
high. DC voltage at the output of the AD8275 is inverted and
gained by the integrator. The inverted signal is injected back
into the REFx pins, nulling the output. In contrast, at high fre-
quencies, the integrator has low gain because the impedance of
C is low. Voltage changes at high frequencies are inverted but at
a low gain. The signal is injected into the REFx pins but it is not
enough to null the output. High frequency signals are, therefore,
allowed to pass.
When a signal exceeds f , HIGH-PASS the AD8275 outputs the
conditioned input signal.
+5V
0.1µF
50kΩ
2
–IN
7
+VS
10kΩ
5
SENSE
OUT
6
50kΩ
3
+IN
20kΩ
8
REF2
20kΩ
1
REF1
AD8275 –VS
4
0.1µF
1
fHIGH-PASS = 2πRC
VOUT
R
C
OP
AMP
+5V
VREF
Figure 42. AC-Coupled Level Translator
AD8275
USING THE AD8275 AS A LEVEL TRANSLATOR IN
A DATA ACQUISITION SYSTEM
Signal size varies dramatically in some data acquisition applica-
tions. Instrumentation amplifiers, such as the AD8253, AD8228,
or AD8221, are often used at the inputs to provide CMRR and
high input impedance. However, the instrumentation amplifiers
output ±10 V signals and the ADC full scale is 5 V or 4.096 V.
In Figure 43, the AD8275 serves as a level translator between
the in-amp and the ADC. The AD8275, along with the AD8228
and the AD8253, have very low gain drift because all gain setting
resistors are internal and laser-trimmed.
+5V
0.1µF
0.1µF
+15V
50kΩ
2
–IN
7
+VS
10kΩ
5
SENSE
0.1µF
OUT
6
IN-AMP
50kΩ
3
+IN
20kΩ
8
REF2
20kΩ
1
REF1
–15V
0.1µF AD8275 –VS
4
33Ω
2.7nF
+IN
VCC
ADC
–IN
REF GND
VREF
10µF
Figure 43. Level Translation in a Data Acquisition System
Rev. A | Page 15 of 16
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