OP37
APPLICATIONS INFORMATION
Noise Measurements
OP37 Series units may be inserted directly into 725 and OP07
sockets with or without removal of external compensation or
nulling components. Additionally, the OP37 may be fitted to
unnulled 741type sockets; however, if conventional 741 nulling
circuitry is in use, it should be modified or removed to ensure
correct OP37 operation. OP37 offset voltage may be nulled to
zero (or other desired setting) using a potentiometer (see offset
nulling circuit).
The OP37 provides stable operation with load capacitances of
up to 1000 pF and
±
10 V swings; larger capacitances should be
decoupled with a 50
Ω
resistor inside the feedback loop. Closed
loop gain must be at least five. For closed loop gain between five
to ten, the designer should consider both the OP27 and the OP37.
For gains above ten, the OP37 has a clear advantage over the
unity stable OP27.
Thermoelectric voltages generated by dissimilar metals at the input
terminal contacts can degrade the drift performance. Best
operation will be obtained when both input contacts are main-
tained at the same temperature.
10k
R
P
V+
To measure the 80 nV peak-to-peak noise specification of the
OP37 in the 0.1 Hz to 10 Hz range, the following precautions
must be observed:
•
The device has to be warmed-up forat least five minutes. As
shown in the warm-up drift curve, the offset voltage typically
changes 4
µV
due to increasing chip temperature after power up.
In the ten second measurement interval, these temperature-
induced effects can exceed tens of nanovolts.
•
For similar reasons, the device has to be well-shielded from
air currents. Shielding minimizes thermocouple effects.
•
Sudden motion in the vicinity of the device can also
“feedthrough” to increase the observed noise.
•
The test time to measure 0.1 Hz to l0 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.
The test time of ten seconds acts as an additional 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.
Optimizing Linearity
–
OP37
+
OUTPUT
V–
Figure 1. Offset Nulling Circuit
Offset Voltage Adjustment
Best linearity will be obtained by designing for the minimum
output current required for the application. High gain and
excellent linearity can be achieved by operating the op amp with
a peak output current of less than
±
10 mA.
Instrumentation Amplifier
INPUT (–)
1
4.7k
1k
POT
4.7k
8
V+
R3
390
R2
100
R4
5k
0.1%
C1
100pF
R6
500
0.1%
Figure 2. TBD
+18V
INPUT (+)
OP37
+
OP37
–18V
Figure 3. Burn-In Circuit
REV. A
–11–
–
+
The input offset voltage of the OP37 is trimmed at wafer level.
However, if further adjustment of V
OS
is necessary, a 10 kΩ trim
potentiometer may be used. TCV
OS
is not degraded (see offset
nulling circuit). Other potentiometer values from 1 kΩ to 1 MΩ
can be used with a slight degradation (0.1
µV/°C
to 0.2
µV/°C)
of
TCV
OS
. Trimming to a value other than zero creates a drift of
approximately (V
OS
/300)
µV/°C.
For example, the change in TCV
OS
will be 0.33
µV/°C
if V
OS
is adjusted to 100
µV.
The offset voltage
adjustment range with a 10 kΩ potentiometer is
±
4 mV. If smaller
adjustment range is required, the nulling sensitivity can be reduced
by using a smaller pot in conjunction with fixed resistors. For
example, the network below will have a
±
280
µV
adjustment range.
A three-op-amp instrumentation amplifier provides high gain and
wide bandwidth. The input noise of the circuit below is 4.9 nV/√Hz.
The gain of the input stage is set at 25 and the gain of the second
stage is 40; overall gain is 1000. The amplifier bandwidth of
800 kHz is extraordinarily good for a precision instrumentation
amplifier. Set to a gain of 1000, this yields a gain bandwidth
product of 800 MHz. The full-power bandwidth for a 20 V p-p
output is 250 kHz. Potentiometer R7 provides quadrature
trimming to optimize the instrumentation amplifier’s ac common-
mode rejection.
R5
500
0.1%
R1
5k
0.1%
R7
100k
R8
20k
0.1%
OP37
–
OP37
+
V
OUT
–
R9
19.8k
R10
500
NOTES:
TRIM R2 FOR A
VCL
= 1000
TRIM R10 FOR dc CMRR
TRIM R7 FOR MINIMUM V
OUT
AT V
CM
= 20V p-p, 10kHz
Figure 4a. TBD
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