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Precision Op Amp Amplifier DC Error Budget Analysis

previous PRECISION OP AMPS
In order to develop a concept for the magnitudes of the various errors in a high precision op amp circuit, a simple room temperature analysis for the OP177F is shown on the opposite page, in Figure 1-88. The amplifier is connected in the inverting mode with a signal gain of 100. The key data sheet specifications are also shown in the diagram. We assume an input signal of 100mV fullscale which corresponds to an output signal of 10V. The various error sources are normalized to fullscale and expressed in parts per million (ppm). Note: parts per million (ppm) error = fractional error × 106 = % error × 104.
Note that the offset errors due to VOS and IOS and the gain error due to finite AVOL can be removed with a system calibration. However, the error due to open-loop gain nonlinearity cannot be removed with calibration and produces a relative accuracy error, often called resolution error.
A second contributor to resolution error is the 1/f noise. This noise is always present and adds to the uncertainty of the measurement. The overall relative accuracy of the circuit at room temperature is 9ppm, equivalent to ~17 bits of resolution.
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Figure 1-88: Precision op amp (OP177F) DC error budget analysis
It is also useful to compare the performance of a number of single-supply op amps to that of the "gold standard" OP177, and this is done in Figure 1-89 below for some representative devices.
Precision single-supply op amp performance characteristics
Figure 1-89: Precision single-supply op amp performance characteristics
Note that the Fig. 1-89 amplifier list does not include the category of chopper op amps, which excel in many of the categories. These are covered separately, immediately below.
Chopper Stabilized Amplifiers
For the lowest offset and drift performance, chopper-stabilized amplifiers may be the only solution. The best bipolar amplifiers offer offset voltages of 25μV and 0.1μV/°C drift. Offset voltages less than 5μV with practically no measurable offset drift are obtainable with choppers, albeit with some penalties.
A basic chopper amplifier circuit is shown in Figure 1-90 below. When the switches are in the "Z" (auto-zero) position, capacitors C2 and C3 are charged to the amplifier input and output offset voltage, respectively. When the switches are in the "S" (sample) position, VIN is connected to VOUT through the path comprised of R1, R2, C2, the amplifier, C3, and R3. The chopping frequency is usually between a few hundred Hz and several kHz, and it should be noted that because this is a sampling system, the input frequency must be much less than one-half the chopping frequency in order to prevent errors due to aliasing. The R1-C1 combination serves as an antialiasing filter. It is also assumed that after a steady state condition is reached, there is only a minimal amount of charge transferred during the switching cycles. The output capacitor, C4, and the load, RL, must be chosen such that there is minimal VOUT droop during the auto-zero cycle.
Classic chopper amplifier
Figure 1-90: Classic chopper amplifier
The basic chopper amplifier of Fig. 1-90 can pass only very low frequencies because of the input filtering required to prevent aliasing. In contrast to this, the chopper-stabilized architecture shown in Figure 1-91 (opposite) is most often used in chopper amplifier implementations. In this circuit, A1 is the main amplifier, and A2 is the nulling amplifier. In the sample mode (switches in "S" position), the nulling amplifier, A2, monitors the input offset voltage of A1 and drives its output to zero by applying a suitable correcting voltage at A1's null pin. Note, however, that A2 also has an input offset voltage, so it must correct its own error before attempting to null A1's offset. This is achieved in the auto-zero mode (switches in "Z" position) by momentarily disconnecting A2 from A1, shorting its inputs together, and coupling its output to its own null pin. During the auto-zero mode, the correction voltage for A1 is momentarily held by C1. Similarly, C2 holds the correction voltage for A2 during the sample mode. In modern IC chopper-stabilized op amps, the storage capacitors C1 and C2 are on-chip.
Note in this architecture that the input signal is always connected to the output, through A1. The bandwidth of A1 thus determines the overall signal bandwidth, and the input signal is not limited to less than one-half the chopping frequency as in the case of the traditional chopper amplifier architecture. However, the switching action does produce small transients at the chopping frequency, that can mix with the input signal frequency and produce intermodulation distortion.
Modern chopper stabilized op amp
Figure 1-91: Modern chopper stabilized op amp
A patented spread-spectrum technique is used in the AD8571/72/74 series of single-supply chopper-stabilized op amps, to virtually eliminate intermodulation effects.
Intermodulation product: fixed versus pseudorandom chopping frequency
Figure 1-92: Intermodulation product: fixed versus pseudorandom chopping frequency
These devices use a pseudorandom chopping frequency swept between 2kHz and 4kHz. Figure 1-92 above compares the intermodulation distortion of a traditional chopper stabilized op amp (AD8551/52/54, left) that uses a fixed 4kHz chopping frequency to that of the AD8571/72/74 (right) that uses the pseudorandom chopping frequency.
A comparison between fixed and pseudorandom chopping on the voltage noise is shown in Figure 1-93 below. Notice for the fixed chopping frequency, there are distinct peaks in the noise spectrum at the odd harmonics of 4kHz, whereas with pseudorandom chopping, the spectrum is much more uniform, although the average noise level is higher.
Voltage noise spectral density comparison: fixed versus pseudorandom chopping frequency
Figure 1-93: Voltage noise spectral density comparison: fixed versus pseudorandom chopping frequency
The AD8571/8572/8574 family of chopper-stabilized op amps offers rail-to-rail input and output single-supply operation, low offset voltage, and low offset drift. As discussed above, the pseudorandom chopping frequency minimizes intermodulation distortion with the input signal. The storage capacitors are internal to the IC, and no external capacitors other than standard decoupling capacitors are required. Key specifications for the devices are given in Figure 1-94 below.
AD8571/72/74 chopper stabilized rail-to-rail input/output amplifiers
Figure 1-94: AD8571/72/74 chopper stabilized rail-to-rail input/output amplifiers
It should be noted that extreme care must be taken when applying all of the chopper stabilized devices. This is because in order to fully realize the full offset and drift performance inherent to the parts, parasitic thermocouple effects in external circuitry must be avoided.
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