OP1177/OP2177/OP4177
+15V
0.1F
500⍀
ADR421
4.12k⍀
4.12k⍀
100⍀
100⍀
20⍀
4.37k⍀
200⍀
؊15V
64
7
5
8 1/2 OP2177
+15V
VOUT
100⍀
RTD
49.9k⍀
5k⍀
؊15V
24
1
3
8 1/2 OP2177
+15V
VOUT
Figure 15. Low Power Linearized RTD Circuit
Single Op Amp Bridge
The low input offset voltage drift of the OP1177 makes it very
effective for bridge amplifier circuits used in RTD signal condi-
tioning. It is often more economical to use a single bridge op amp
as opposed to an instrumentation amplifier.
In the circuit of Figure 16, the output voltage at the op amp is:
( ) VO
=
R2
R
V
REF
δ
R1
R
+
1
+
R1
R 2
1+ δ
where δ = ∆R/R is the fractional deviation of the RTD resis-
tance with respect to the bridge resistance due to the change in
temperature at the RTD.
For δ << 1, the expression above becomes:
VO
≅
R2
R VREF
1 +
δ
R1 +
R
R1
R2
=
R2
R
1
+
R1
R 2
+
R
R
1
2
V
REF
δ
With VREF constant, the output voltage is linearly proportional to
δ with a gain factor of:
V REF
R2
R
1 +
R1
R 2
+
R1
R
2
0.1F
15V
ADR421
R
R
R(1+␦)
R
RF
V؊
24
1
VOUT
3
7 OP1177
V+
RF
REALIZATION OF ACTIVE FILTERS
Bandpass KRC or Sallen-Key Filter
The low offset voltage and the high CMRR of the OP1177 make
it an excellent choice for precision filters such as the KRC filter
shown in Figure 17. This filter type offers the capability to tune
the gain and the cutoff frequency independently.
Since the common-mode voltage into the amplifier varies with the
input signal in the KRC filter circuit, a high CMRR is required to
minimize distortion. Also, the low offset voltage of the OP1177 allows
a wider dynamic range when the circuit gain is chosen to be high.
The circuit of Figure 17 consists of two stages. The first stage is
a simple high-pass filter whose corner frequency fC is:
1
2π C 1C 2R1R 2
(2)
and whose
Q = K R1
(3)
R2
where K is the dc gain.
Choosing equal capacitor values minimizes the sensitivity and
simplifies Equation 2 to:
1
2πC R1R 2
The value of Q determines the peaking of the gain versus frequency
(ringing in transient response). Commonly chosen values for Q
are generally near unity.
Setting Q =
1
2,
yields minimum gain peaking and minimum ringing.
Determine values for R1 and R2 by use of Equation 3.
For Q =
1
, R1/R2 = 2 in the circuit example. Pick R1 = 5 kΩ
2
and R2 = 10 kΩ for simplicity.
The second stage is a low-pass filter whose corner frequency can
be determined in a similar fashion. For R3 = R4 = R.
fC =
1
2πR C 3
and Q = 1
2
C3
C4
C4
Channel Separation
Multiple amplifiers on a single die are often required to reject
any signals originating from the inputs or outputs of adjacent
channels. OP2177 input and bias circuitry is designed to prevent
feedthrough of signals from one amplifier channel to the other. As
a result the OP2177 has an impressive channel separation of
greater than –120 dB for frequencies up to 100 kHz and greater
than –115 dB for signals up to 1 MHz.
REV. B
Figure 16. Single Bridge Amplifier
–15–