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Amplifier Techniques that Enable Precision Performance
Choosing lower-impedance feedback and source impedances and including output filtering can virtually eliminate both offset shift and noise tone issues. The application note, “Optimizing Chopper Amplifier Accuracy,” explains the topic in detail and provides design recommendations.
What Factors are Reducing the Cost of Precision Analog?
Zero-drift and e-trim technologies are two design approaches that help reduce component and system costs. Both use CMOS devices because they require digital control or communications. For zero-drift devices, the savings occur since no trimming is needed, as the device employs a fixed internal calibration scheme to minimize offset. Laser trimming can be fairly expensive because it’s a time-consuming iterative semiconductor test process.
The test time for semiconductor devices accounts for a significant portion of device costs — semiconductor testers are expensive, and that expense needs to be amortized across the life of the tester. Zero-drift devices require no trimming. The trimming time for e-trim devices will increase test times somewhat compared to zero-drift devices. However, the test-time increase for e-trim devices is minimal because the process uses a digital command to the device, whereas laser trimming is an iterative series of laser cuts in the resistor, which takes more time than e-trim.
As a general trend, the wafer size in modern wafer fabrication plants has increased, while there’s been a decrease in die size. This yields more die manufactured per wafer, which also helps lower costs. In some cases, reducing die size with some performance compromise can further optimize costs.
For example, the output transistors on most amplifiers take up a large portion of the die area. Reducing these transistors can significantly reduce costs, but there will be some compromise on output current and output swing to the rail. In general, eliminating features by making tradeoffs on specifications can help shrink die size and cost.
How Can Precision Analog Save Money?
Even with the substantially reduced cost of some modern precision devices, many designers will still choose the lowest-cost general-purpose devices. This can be a reasonable approach to lowering cost, but the general-purpose device will have significant accuracy tradeoffs.
For example, the TLV888 from TI is a precision cost-optimized zero-drift device, while TI’s TL071 is a general-purpose device first released in 1978. The cost of both devices is quite low, but the general-purpose device is roughly half the cost of the cost-optimized precision device.
Figure 5 summarizes the primary specifications for these two devices. The precision device is hundreds of times better than the general-purpose device for VOS, VOS drift, and other parameters. In some cases, the specifications may not be a priority — cost is the main target. Keep in mind, however, that there may be some hidden expenses in using less-precise devices.
