previous Op Amp Process Technologies
In this section, basic op amp specifications are discussed. The importance of any given specification depends of course upon the application. For instance offset voltage, offset voltage drift, and open-loop gain are very critical in precision sensor signal conditioning circuits, but not as important in high speed applications where bandwidth, slew rate, and distortion are the key specifications.
Most op amp specifications are largely topology independent. However, although voltage feedback and current feedback op amps have similar error terms and specifications, the application of each part warrants discussing some of the specifications separately. In the following discussions, this will be done where significant differences exist.
Input Offset Voltage, VOS
Ideally, if both inputs of an op amp are at exactly the same voltage, then the output should be at zero volts. In practice, a small differential voltage must be applied to the inputs to force the output to zero. This is known as the input offset voltage, VOS.
Input offset voltage
Figure 1-37: Input offset voltage
Input offset voltage is modeled as a voltage source, VOS, in series with the inverting input terminal of the op amp as shown in Figure 1-37 above. The corresponding output offset voltage (due to VOS) is obtained by multiplying the input offset voltage by the DC noise gain of the circuit (see Fig. 1-3 and Eq. 1-2, again).
Chopper stabilized op amps have a VOS which is less than 1μV (AD8551 series), and the best precision bipolar op amps (super-beta or bias stabilized) can have offsets as low as 25μV (OP177F). The very best trimmed FET types have about 100μV of offset (AD8610B), and untrimmed CMOS op amps can range from 5 to 50mV. However, the ADI DigiTrim™ CMOS op amps have offset voltages less than 1mV (AD8605). Generally speaking, "precision" op amps will have VOS < 0.5mV, although some high speed amplifiers may be a little worse than this.
Measuring input offset voltages of a few microvolts requires that the test circuit does not introduce more error than the offset voltage itself. Figure 1-38 below shows a standard circuit for measuring offset voltage. The circuit amplifies the input offset voltage by the noise gain of 1001. The measurement is made at the amplifier output using an accurate digital voltmeter. The offset referred to the input (RTI) is calculated by dividing the output voltage by the noise gain. The small source resistance seen by the inputs results in negligible bias current contribution to the measured offset voltage. For example, 2nA bias current flowing through the 10Ω resistor produces a 0.02μV error referred to the input.
Measuring input offset voltage
Figure 1-38: Measuring input offset voltage
As simple as this circuit looks, it can give inaccurate results when testing precision op amps, unless care is taken in implementation. The largest potential error source comes from parasitic thermocouple junctions, formed where two different metals are joined. This thermocouple voltage can range from 2μV/ºC to more than 40μV/ºC. Note that in this circuit additional "dummy" resistors have been added to the non-inverting input, in order to exactly match/balance the thermocouple junctions in the inverting input path.
The accuracy of the measurement also depends on the mechanical layout of the components and exactly how they are placed on the PC board. Keep in mind that the two connections of a component such as a resistor create two equal, but opposite polarity thermoelectric voltages (assuming they are connected to the same metal, such as the copper trace on a PC board). These will cancel each other, assuming both are at exactly the same temperature. Clean connections and short lead lengths help to minimize temperature gradients and increase the accuracy of the measurement.
In the test circuit, airflow should be minimal so that all the thermocouple junctions stabilize at the same temperature. In some cases, the circuit should be placed in a small closed container to eliminate the effects of external air currents. The circuit should be placed flat on a surface so that convection currents flow up and off the top of the board, not across the components, as would be the case if the board were mounted vertically.
Measuring the offset voltage shift over temperature is an even more demanding challenge. Placing the printed circuit board containing the amplifier being tested in a small box or plastic bag with foam insulation prevents the temperature chamber air current from causing thermal gradients across the parasitic thermocouples. If cold testing is required, a dry nitrogen purge is recommended. Localized temperature cycling of the amplifier itself using a Thermostream-type heater/cooler may be an alternative, however these units tend to generate quite a bit of airflow that can be troublesome.
Alternate input offset voltage measurement using an in-amp
Figure 1-39: Alternate input offset voltage measurement using an in-amp
Generally, the test circuit of Fig. 1-38 can be made to work for many amplifiers. Low absolute values for the small resistors (such as 10Ω) will minimize bias current induced errors. An alternate VOS measurement method is shown in Fig. 1-39, and is suitable for cases of high and/or unequal bias currents (as in the case of current feedback op amps).
In this measurement method, an in-amp is connected to the op amp input terminals through isolation resistors, and provides the gain for the measurement. The offset voltage of the in-amp (measured with S closed) must then be subtracted from the final VOS measurement. Also, the circuit shown below in Figure 1-44 for measuring input bias currents can also be used to measure input offset voltage independent of bias currents.
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