LT1028_15 Datasheet, PDF (14/28 Pages) Linear Technology – Ultralow Noise Precision High Speed Op Amps

LT1028/LT1128
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.
1k
1
2_– 8
INPUT
3
LT1028
LT1128
+₄
76
15V
OUTPUT
–15V
Figure 6
1028 F06
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.
10k*
15V
2_{– 7}
200Ω*
3
LT1028
LT1128
+₄
6
V_O
10k*
–15V
V_O = 100V_OS
* RESISTORS MUST HAVE LOW
THERMOELECTRIC POTENTIAL
1028 F07
Figure 7. Test Circuit for Offset Voltage
and Offset Voltage Drift with Temperature
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).
R_F
–
OUTPUT
+
Figure 8
6V/µs
1028 F08
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.
1028fc
14
For more information www.linear.com/LT1028