AD8275
INCREASING INPUT IMPEDANCE
USING THE AD8275 AS A LEVEL TRANSLATOR IN
A DATA ACQUISITION SYSTEM
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.
+5V
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.
0.1µF
+13V
8
INVERTING
INPUT
7
+V
0.1µF
1
S
3
2
50kΩ
–IN
10kΩ
SENSE
OUT
2
5
6
AD8620
1/2
+5V
V
OUT
0.1µF
0.1µF
V
REF
7
50kΩ
+IN
20kΩ
REF2
REF1
6
5
3
8
1
AD8620
2/2
+V
S
7
20kΩ
50kΩ
–IN
10kΩ
2
5
NON-
INVERTING
INPUT
0.1µF
SENSE
AD8275 –V
S
4
33Ω
VCC
ADC
4
+IN
–IN
+15V
–13V
OUT
6
0.1µF
0.1µF
2.7nF
REF GND
Figure 41. Adding Op Amp Buffers for High Input Impedance
50kΩ
+IN
20kΩ
20kΩ
3
8
1
IN-AMP
REF2
REF1
AC COUPLING
VREF
10µF
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.
–15V
AD8275 –V
S
4
Figure 43. Level Translation in a Data Acquisition System
When a signal exceeds fHIGH-PASS, the AD8275 outputs the
conditioned input signal.
+5V
0.1µF
7
+V
S
50kΩ
–IN
10kΩ
1
2
5
6
f
=
HIGH-PASS
2πRC
SENSE
OUT
V
OUT
R
C
50kΩ
+IN
20kΩ
20kΩ
3
8
1
REF2
REF1
OP
AMP
AD8275 –V
S
V
4
REF
0.1µF
+5V
Figure 42. AC-Coupled Level Translator
Rev. A | Page 15 of 16
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