TLC27M4_15 Datasheet, PDF (18/52 Pages) Texas Instruments – PRECISION QUAD OPERATIONAL AMPLIFIERS

TLC27M4, TLC27M4A, TLC27M4B, TLC27M4Y, TLC27M9
SLOS093D – OCTOBER 1987 – REVISED OCTOBER 2012
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PARAMETER MEASUREMENT INFORMATION (continued)
Input Bias Current
Because of the high input impedance of the TLC27M4 and TLC27M9 operational amplifiers, attempts to measure
the input bias current can result in erroneous readings. The bias current at normal room ambient temperature is
typically less than 1 pA, a value that is easily exceeded by leakages on the test socket. Two suggestions are
offered to avoid erroneous measurements:
1. Isolate the device from other potential leakage sources. Use a grounded shield around and between the
device inputs (see Figure 4). Leakages that would otherwise flow to the inputs are shunted away.
2. Compensate for the leakage of the test socket by actually performing an input bias current test (using a
picoammeter) with no device in the test socket. The actual input bias current can then be calculated by
subtracting the open-socket leakage readings from the readings obtained with a device in the test socket.
One word of caution—many automatic testers as well as some bench-top operational amplifier testers use the
servo-loop technique with a resistor in series with the device input to measure the input bias current; the voltage
drop across the series resistor is measured and the bias current is calculated. This method requires that a device
be inserted into the test socket to obtain a correct reading; therefore, an open-socket reading is not feasible
using this method.
7
1
V = V_IC
8
14
Figure 4. Isolation Metal Around Device Inputs
(J and N packages)
Low-Level Output Voltage
To obtain low-supply-voltage operation, some compromise was necessary in the input stage. This compromise
results in the device low-level output being dependent on both the common-mode input voltage level as well as
the differential input voltage level. When attempting to correlate low-level output readings with those quoted in
the electrical specifications, these two conditions should be observed. If conditions other than these are to be
used, please refer to Figure 14 through Figure 19 in the Typical Characteristics of this data sheet.
Input Offset Voltage Temperature Coefficient
Erroneous readings often result from attempts to measure temperature coefficient of input offset voltage. This
parameter is actually a calculation using input offset voltage measurements obtained at two different
temperatures. When one (or both) of the temperatures is below freezing, moisture can collect on both the device
and the test socket. This moisture results in leakage and contact resistance, which can cause erroneous input
offset voltage readings. The isolation techniques previously mentioned have no effect on the leakage since the
moisture also covers the isolation metal itself, thereby rendering it useless. It is suggested that these
measurements be performed at temperatures above freezing to minimize error.
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