AD8275
Rev. A | Page 15 of 16
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.
07546-036
AD8275
7
4
5
6
8
1
2
1
3
2
7
6
5
4
8
50kΩ
0.1µF
50kΩ
20kΩ
20kΩ
10kΩ
3
–IN
1/2
2/2
+IN
REF2
V
REF
REF1
–V
S
–13V
0.1µF
0.1µF
+V
S
+5V
+13V
OUT
V
OUT
SENSE
INVERTING
INPUT
NON-
INVERTING
INPUT
AD8620
AD8620
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-PAS S
, the AD8275 outputs the
conditioned input signal.
07546-037
AD8275
7
4
5
6
8
2
50kΩ
0.1µF
0.1µF
50kΩ
20kΩ
20kΩ
R
C
10kΩ
3
+IN
–IN
REF2
REF1
V
REF
–V
S
+V
S
+5V
+5V
OUT
V
OUT
SENSE
V
OUT
1
f
HIGH-PASS
=
1
2πRC
OP
AMP
Figure 42. AC-Coupled Level Translator
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.
07546-143
VREF
AD8275
7
4
5
6
8
2
50kΩ
0.1µF
50kΩ
20kΩ
20kΩ
33Ω
10kΩ
3
+IN
–IN
REF2
REF1
–V
S
+V
S
+5V
OUT
SENSE
0.1µF
2.7nF
10µF
1
ADC
VCC
GNDREF
+IN
–IN
0.1µF
+15V
–15V
0.1µF
IN-AMP
Figure 43. Level Translation in a Data Acquisition System
