Datasheet
13
LT1677
1677fa
APPLICATIO S I FOR ATIO
WUUU
Offset Voltage and Drift
Thermocouple effects, caused by temperature gradients
across dissimilar metals at the contacts to the input
terminals, can exceed the inherent drift of the amplifier
unless proper care is exercised. Air currents should be
minimized, package leads should be short, the two input
leads should be close together and maintained at the same
temperature.
The circuit shown to measure offset voltage is also used as
the burn-in configuration for the LT1677, with the supply
voltages increased to ±20V (Figure 4).
Figure 4. Test Circuit for Offset Voltage and
Offset Voltage Drift with Temperature
1677 F04
V
OUT
V
OUT
=
1000V
OS
*RESISTORS MUST HAVE LOW
THERMOELECTRIC POTENTIAL
7
6
4
2
3
15V
–15V
–
+
LT1677
50k*
100Ω*
50k*
As with all operational amplifiers when R
F
> 2k, a pole will
be created with R
F
and the amplifier’s input capacitance,
creating additional phase shift and reducing the phase
margin. A small capacitor (20pF to 50pF) in parallel with R
F
will eliminate this problem.
Noise Testing
The 0.1Hz to 10Hz peak-to-peak noise of the LT1677 is
measured in the test circuit shown (Figure 6a). The fre-
quency response of this noise tester (Figure 6b) indicates
that the 0.1Hz corner is defined by only one zero. The test
time to measure 0.1Hz to 10Hz noise should not exceed
ten seconds, as this time limit acts as an additional zero to
eliminate noise contributions from the frequency band
below 0.1Hz.
Measuring the typical 90nV peak-to-peak noise perfor-
mance of the LT1677 requires special test precautions:
1. The device should be warmed up for at least five
minutes. As the op amp warms up, its offset voltage
changes typically 3µV due to its chip temperature
increasing 10°C to 20°C from the moment the power
supplies are turned on. In the ten-second measurement
interval these temperature-induced effects can easily
exceed tens of nanovolts.
2. For similar reasons, the device must be well shielded
from air currents to eliminate the possibility of
thermoelectric effects in excess of a few nanovolts,
which would invalidate the measurements.
3. Sudden motion in the vicinity of the device can also
“feedthrough” to increase the observed noise.
Current noise is measured in the circuit shown in Figure 7
and calculated by the following formula:
i
e
nV
M
n
no
=
()
−
()
⎡
⎣
⎢
⎤
⎦
⎥
()()
2
2
12
130
101
1 101
•
/
Ω
The LT1677 achieves its low noise, in part, by operating
the input stage at 100µA versus the typical 10µA of most
other op amps. Voltage noise is inversely proportional
while current noise is directly proportional to the square
Figure 5. Pulsed Operation
1677 F05
LT1677
–
+
R
F
OUTPUT
2.5V/µs
Unity-Gain Buffer Application
When R
F
≤ 100Ω and the input is driven with a fast, large-
signal pulse (>1V), the output waveform will look as
shown in the pulsed operation diagram (Figure 5).
During the fast feedthrough-like portion of the output, the
input protection diodes effectively short the output to the
input and a current, limited only by the output short-circuit
protection, will be drawn by the signal generator. With
R
F
≥ 500Ω, the output is capable of handling the current
requirements (I
L
≤ 20mA at 10V) and the amplifier stays
in its active mode and a smooth transition will occur.