Datasheet

LT1028/LT1128
14
1028fb
Figure 7. Test Circuit for Offset Voltage
and Offset Voltage Drift with Temperature
–
+
R
F
1028 F08
OUTPUT
6V/µs
–
+
–15V
10k*
200Ω*
LT1028
LT1128
1028 F07
10k*
V
O
= 100V
OS
* RESISTORS MUST HAVE LOW
THERMOELECTRIC POTENTIAL
V
O
6
7
2
4
3
15V
applicaTions inForMaTion
General
The LT1028/LT1128 series devices may be inserted directly
into OP-07, OP-27, OP-37, LT1007 and LT1037 sockets
with or without removal of external nulling components. In
addition, the LT1028/LT1128 may be fitted to 5534 sockets
with the removal of external compensation components.
Offset Voltage Adjustment
The input offset voltage of the LT1028/LT1128 and its drift
with temperature, are permanently trimmed at wafer test-
ing to a low level. However, if further adjustment of V
OS
is
necessary, the use of a 1k nulling potentiometer will not
degrade drift with temperature. Trimming to a value other
than zero creates a drift of (V
OS
/300)µV/°C, e.g., if V
OS
is adjusted to 300µV, the change in drift will be 1µV/°C.
The adjustment range with a 1k pot is approximately
±1.1mV.
Unity-Gain Buffer Applications (LT1128 Only)
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 8).
–
+
6
1k
INPUT
LT1028
LT1128
1028 F06
7
8
1
2
3
4
OUTPUT
–15V
15V
Figure 6
Figure 8
Offset Voltage and Drift
Thermocouple effects, caused by temperature gradients
across dissimilar metals at the contacts to the input termi-
nals, 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 in Figure 7 to measure offset voltage
is also used as the burn-in configuration for the LT1028/
LT1128.
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.
As with all operational amplifiers when R
F
> 2k, a pole will
be created with RF 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.