Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
2000-03-10
2001-07-31
Brown, Glenn W. (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C330S002000, C330S069000, C330S260000
Reexamination Certificate
active
06268734
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of operational amplifier test circuits, and particularly to test circuits which determine an op amp's input offset voltage (VOS) and input bias current (IB).
2. Description of the Related Art
The parameters of an operational amplifier (op amp) are typically established by means of a test circuit which applies known inputs to the op amp's inverting and non-inverting inputs and monitors its output. For example, input offset voltage (VOS), i.e., the voltage that results in an output of zero when applied to both op amp inputs, is typically determined by connecting the inverting and non-inverting inputs of the device under test (DUT) to ground via respective resistors, and connecting a feedback resistor between the DUT's output and its inverting input. The input and feedback resistors are generally selected to establish a high gain around the DUT. VOS for the DUT is determined by measuring its output voltage under these conditions, and dividing the measured voltage by the gain value.
Another frequently tested op amp parameter is input bias current (IB), i.e., the current which flows into each of the DUT's inputs when it is operating. This can be determined in any of several ways. For example, a current meter may be connected in series with each of the DUT's inputs and IB measured directly. Alternatively, IB can be determined by measuring the voltage across a pair of input resistors with a very sensitive voltmeter.
Many op amps must be tested for both VOS and IB. When this is the case, IB can be determined by measuring VOS as described above, and then inserting an additional resistance in series with one of the DUT's inputs and measuring VOS again. The difference between the two VOS readings is divided by the additional resistance to give IB.
Virtually all of these test strategies require the use of some sort of switching circuitry, to manipulate the test conditions and to make appropriate measurements as necessary. For example, when IB is measured by comparing VOS measurements with and without the presence of an additional input resistance, a switching means must be used to insert the resistance into and remove it from the circuit. Semiconductor switches can be used for this purpose, but doing so tends to degrade the accuracy of the testing, due to the inherent resistance of such a switch, together with the time required to fully turn on or off, and their leakage current in the off condition. While this may not be a problem for general purpose op amps, it is unacceptable when testing precision op amps, which can have VOS and IB specifications of less than 10 &mgr;V and 1 nA, respectively.
Conventional electromagnetic relays are often employed as a switching means for testing precision op amps, due to the very low resistance conductive path provided when their contacts are closed. However, when the electromagnetic coil of a conventional relay is energized for a length of time, heat from the coil generates a thermal E.M.F. in series with the relay contacts. This thermal voltage represents an error that gets added directly to the DUT's VOS measurement, degrading the accuracy of both VOS and IB measurements. When very low VOS and/or IB values must be measured, the error due to thermal E.M.F. can be unacceptably high.
SUMMARY OF THE INVENTION
An op amp test circuit is presented which overcomes the difficulties noted above. The invention avoids the testing inaccuracies introduced by conventional switching means, thus enabling highly accurate testing of precision op amps.
The invention requires that the switching means for a DUT test circuit comprise latching relays, the contacts of which are latched open or closed as necessary, preferably prior to making test measurements. The use of metal relay contacts ensures the provision of a conductive path with near-zero resistance when the contacts are closed. In addition, the electromagnetic coil of a latching relay need be energized only briefly to latch the contacts in a desired position, after which power to the coil can be removed. By removing power to the relay coil except when changing its state, the heating duty cycle is kept low, and the accuracy-degrading thermal E.M.F. present in the prior art is virtually eliminated.
The novel test circuit is advantageously used for testing both VOS and IB for high precision, low-VOS op amps. In a preferred embodiment, first and second resistances are connected in series with an op amp's inverting and non-inverting inputs, respectively, with a set of latching relay contacts connected across each resistance. Additional resistors around the DUT provide a high loop gain value. The coils controlling each set of contacts are briefly energized to latch the contacts closed, thereby keeping thermal E.M.F. negligible. With the input resistances bypassed by the closed contacts, the DUT's VOS is determined. One set of contacts is then opened (by briefly energizing the appropriate latching relay coil), inserting its corresponding resistance into the DUT's input circuit and causing VOS to change. The difference between the two VOS values is proportional to the IB value for the corresponding input.
REFERENCES:
patent: 3783372 (1974-01-01), Boyd
patent: 5103122 (1992-04-01), O'leary et al.
patent: 5202613 (1993-04-01), Kruse
patent: 5327029 (1994-07-01), Ericson et al.
patent: 5386160 (1995-01-01), Archer et al.
patent: 5592124 (1997-01-01), Mullins et al.
patent: 5744965 (1998-04-01), Miller et al.
patent: 6081106 (1998-08-01), Camerlo
Analog Devices Inc.
Brown Glenn W.
Hamdan Wasseem H.
Koppel & Jacobs
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