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Noise Considerations for Chopper-Stabilized Op Amps

previous Precision Op Amp Amplifier DC Error Budget Analysis
It is interesting to consider the effects of a chopper amplifier on low frequency 1/f noise. If the chopping frequency is considerably higher than the 1/f corner frequency of the input noise, the chopper-stabilized amplifier continuously nulls out the 1/f noise on a sample-by-sample basis. Theoretically, a chopper op amp therefore has no 1/f noise. However, the chopping action produces wideband noise which is generally much worse than that of a precision bipolar op amp.
Figure 1-95 below shows the noise of a precision bipolar amplifier (OP177) versus that of the AD8571/72/74 chopper-stabilized op amp. The peak-to-peak noise in various bandwidths is calculated for each in the table below the graphs.
Noise: bipolar versus chopper stabilized op amp
Figure 1-95: Noise: bipolar versus chopper stabilized op amp
Note from the data that as the frequency is lowered, the chopper amplifier noise continues to drop, while the bipolar amplifier noise approaches a limit determined by the 1/f corner frequency and its white noise. Notice that only at very low frequencies (<0.01Hz ) is the chopper noise performance superior to that of the bipolar op amp.
In order to take advantage of the chopper op amp's lack of 1/f noise, much filtering is required— otherwise the total noise of a chopper will always be worse than a good bipolar op amp. Choppers should therefore be selected on the basis of their low offset and drift— not because of their lack of 1/f noise.
Introduction to HIGH SPEED OP AMPS
High speed analog signal processing applications, such as video and communications, require op amps which have wide bandwidth, fast settling time, low distortion and noise, high output current, good DC performance, and operate at low supply voltages. These devices are widely used as gain blocks, cable drivers, ADC pre-amps, current-to-voltage converters, etc. Achieving higher bandwidths for less power is extremely critical in today's portable and battery-operated communications equipment. The rapid progress made over the last few years in high speed linear circuits has hinged not only on the development of IC processes but also on innovative circuit topologies.
Amplifier bandwidth versus supply current for Analog Devices' processes
Figure 1-96: Amplifier bandwidth versus supply current for Analog Devices' processes
The evolution of high speed processes using amplifier bandwidth as a function of supply current as a figure of merit is shown in Figure 1-96 above. (In the case of duals, triples, and quads, the current per amplifier is used). Analog Devices BiFET process, which produced the AD712 (3MHz bandwidth, 3mA current, yields about 1MHz per mA.
The CB (Complementary Bipolar) process (AD817, AD847, AD811, etc.) yields about 10MHz/mA of supply current. The fts of the CB process PNP transistors are about 700MHz, and the NPN's about 900MHz. The CB process at Analog Devices was introduced in 1985.
The next complementary bipolar process from Analog Devices is a high speed dielectrically isolated process called "XFCB" (eXtra Fast Complementary Bipolar) which was introduced in 1992 .This process yields 3GHZ PNPs and 5GHZ matching NPNs, and coupled with innovative circuit topologies allows op amps to achieve new levels of cost-effective performance at astonishing low quiescent currents. The approximate figure of merit for this process is typically 100MHz/mA, although the AD8011 op amp is capable of 300MHz bandwidth on 1mA of supply current due to its unique two-stage current-feedback architecture described later in this section.
Even faster CB processes have been developed at Analog Devices for low voltage supply products such as "XFCB 1.5" (5GHz PNP, 9GHz NPN), and "XFCB 2" (9GHZ PNP, 16GHz NPN). The AD8351 differential low distortion RF amplifier (shown on Fig. 1-96) is fabricated on "XFCB 1.5" and has a bandwidth of 2GHz for a gain of 12dB. It is expected that newer complementary bipolar processes will be optimized for higher fts.
In order to select intelligently the correct high speed op amp for a given application, an understanding of the various op amp topologies as well as the tradeoffs between them is required. The two most widely used topologies are voltage feedback (VFB) and current feedback (CFB). An overview of these topologies has been presented in a previous section, but the following discussion treats the frequency-related aspects of the two topologies in considerably more detail.
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