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TLC27 Datasheet(PDF) 55 Page - Texas Instruments |
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TLC27 Datasheet(HTML) 55 Page - Texas Instruments |
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55 / 74 page  TLC271, TLC271A, TLC271B LinCMOS ™ PROGRAMMABLE LOW-POWER OPERATIONAL AMPLIFIERS SLOS090C – NOVEMBER 1987 – REVISED AUGUST 1997 55 POST OFFICE BOX 655303 • DALLAS, TEXAS 75265 PARAMETER MEASUREMENT INFORMATION input bias current Because of the high input impedance of the TLC271 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 101). 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 us 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. V = VIC 4 1 5 8 Figure 101. Isolation Metal Around Device inputs (JG and P packages) low-level output voltage To obtain low-supply-voltage operation, some compromise is 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 the Typical Characteristics section 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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