5962-8688701CA View Datasheet(PDF) - Analog Devices
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5962-8688701CA Datasheet PDF : 16 Pages
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OP227
BASIC CONNECTIONS
V+(A)
10k⍀
2 1 14
(–)
3
INPUTS
(+)
4
(+) 11
INPUTS
(–) 10
A
OP227
B
13
OUT (A)
12
V–(A)
5
V–(B)
6
OUT (B)
98
7
10k⍀
V+(A)
Figure 1. Offset Nulling Circuit
APPLICATIONS INFORMATION
Noise Measurements
To measure the 80 nV peak-to-peak noise specification of the
OP227 in the 0.1 Hz to 10 Hz range, the following precautions
must be observed:
• The device must be warmed up for at least five minutes. As
shown in the warm-up drift curve, the offset voltage typically
changes 4 mV due to increasing chip temperature after power-up.
In the 10-second measurement interval, these temperature-
induced effects can exceed tens-of-nanovolts.
∑ For similar reasons, the device must be well shielded from air
currents. Shielding minimizes thermocouple effects.
∑ Sudden motion in the vicinity of the device can also “feed-
through” to increase the observed noise.
∑ The test time to measure 0.1 Hz to 10 Hz noise should not
exceed 10-seconds. As shown in the noise-tester frequency-
response curve, the 0.1 Hz corner is defined by only one zero
to eliminate noise contributions from the frequency band
below 0.1 Hz.
∑ A noise-voltage-density test is recommended when measuring
noise on a large number of units. A 10 Hz noise-voltage-
density measurement will correlate well with a 0.1 Hz to 10 Hz
peak-to-peak noise reading, since both results are determined
by the white noise and the location of the 1/f corner frequency.
Instrumentation Amplifier Applications of the OP227
The excellent input characteristics of the OP227 make it ideal
for use in instrumentation amplifier configurations where low
level differential signals are to be amplified. The low noise, low
input offsets, low drift, and high gain, combined with excellent
CMR provide the characteristics needed for high performance
instrumentation amplifiers. In addition, CMR versus frequency
is very good due to the wide gain bandwidth of these op amps.
The circuit of Figure 2 is recommended for applications where
the common-mode input range is relatively low and differential
gain will be in the range of 10 to 1000. This two op amp
instrumentation amplifier features independent adjustment of
common-mode rejection and differential gain. Input imped-
ance is very high since both inputs are applied to non-inverting
op amp inputs.
R0
R1
R2
VCM – 1/2Vd
R4
A1
V1
R3
VCM + 1/2Vd
A2
VO
[ ( ) ] ( ) VO
=
R4
R3
1+
1
2
R2
R1
+
R3
R4
+
R2 + R3
R0
Vd
+
R4
R3
R3
R4
–
R2
R1
VCM
Figure 2. Two Op Amp Instrumentation Amplifier Configuration
The output voltage VO, assuming ideal op amps, is given in
Figure 2. the input voltages are represented as a common-mode
input, VCM, plus a differential input, Vd. The ratio R3/R4 is
made equal to the ratio R2/R1 to reject the common mode input
VCM. The differential signal VO is then amplified according to:
VO
=
R4
R3
ÊËÁ1
+
R3
R4
+
R2 + R3
RO
ˆ
¯˜
Vd
,
where
R3
R4
=
R2
R1
Note that gain can be independently varied by adjusting RO.
From considerations of dynamic range, resistor tempco match-
ing, and matching of amplifier response, it is generally best to
make R1, R2, R3, and R4 approximately equal. Designing R1,
R2, R3, and R4 as RN allows the output equation to be further
simplified:
V
O
Ê
= 2Á1 +
Ë
RN
R
O
ˆ
˜ Vd , where
¯
RN
= R1
= R2
= R3
= R4
–10–
REV. A
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