Discrete Difference Amplifiers vs. Integrated Solutions

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Discrete Difference Amplifiers vs. Integrated Solutions [Copy link]

Question:

Why spend more to get less done?



Answer:

The classic discrete difference amplifier design is very simple. What's the complication with an op amp and a four-resistor network?

However, the performance of this circuit may not be as good as the designer wants. Starting from an actual production design, this article discusses some of the disadvantages associated with discrete resistors, including gain accuracy, gain drift, ac common-mode rejection (CMR), and offset drift. The

classic four-resistor difference amplifier is shown in Figure 1.


Figure 1. Classic discrete difference amplifier

The transfer function of this amplifier circuit is:



If R1 = R3 and R2 = R4, then Equation 1 simplifies to:



This simplification is useful for quickly estimating the expected signal, but these resistors are never exactly equal. In addition, resistors often have low precision and high temperature coefficients, which can introduce significant errors into the circuit.

For example, using a good op amp and standard 1%, 100 ppm/°C gain setting resistors, the initial gain error can be as high as 2%, and the temperature drift can be as high as 200ppm/°C. To solve this problem, one solution is to use a monolithic resistor network for precision gain setting, but this structure is large and expensive. In addition to low precision and significant temperature drift, most discrete differential op amp circuits also have poor CMR and an input voltage range that is less than the supply voltage. In addition, monolithic instrumentation amplifiers will have gain drift because the internal resistor network of the preamplifier does not match the external gain setting resistors connected to the RG pin.

The best way to solve all of these problems is to use a difference amplifier with internal gain setting resistors, such as the AD8271. Typically, these products consist of a high-precision, low-distortion op amp and multiple trimmed resistors. By connecting these resistors, a wide variety of amplifier circuits can be created, including differential, non-inverting, and inverting configurations. The resistors on the chip can be connected in parallel to provide a wider range of options. Using on-chip resistors provides designers with several advantages over discrete designs. The

DC performance of an op amp circuit is largely determined by the accuracy of the surrounding resistors. These internal resistors are laid out to match closely and are laser trimmed and tested for matching accuracy. Therefore, it guarantees that the characteristics such as gain drift, common-mode rejection, and gain error are highly accurate. The circuit shown in Figure 1 can be integrated to provide 0.1% gain accuracy and less than 10 ppm/°C gain drift, as shown in Figure 2.


Figure 2. Gain Error vs. Temperature - AD8271 vs. Discrete Solution

AC Performance

In terms of circuit size, integrated circuits are much smaller than printed circuit boards (PCBs), so the corresponding parasitic parameters are also smaller, which is beneficial to AC performance. For example, the positive and negative inputs of the AD8271 op amp intentionally do not have output pins. These nodes are not connected to the traces on the PCB, and the capacitance is kept low, which improves loop stability and optimizes common-mode rejection over the entire frequency range. See Figure 3 for a performance comparison. Figure 3. CMRR


vs. Frequency - AD8271 vs. Discrete Solution CMRR Comparison

An important function of a difference amplifier is to reject the common-mode signal at both inputs. Referring to Figure 1, if resistors R1 to R4 are not perfectly matched (or the ratios of R1, R2 and R3, R4 are not matched when the gain is greater than 1), then part of the common-mode voltage will be amplified by the difference amplifier and appear at VOUT as an effective differential voltage between V1 and V2, which cannot be distinguished from the actual signal. If the resistors are not ideal, some of the common-mode voltage will be amplified by the difference amplifier and appear at VOUT as an effective differential voltage between V1 and V2, which is indistinguishable from the actual signal. The ability of the difference amplifier to reject this portion of the voltage is called common-mode rejection. This parameter can be expressed as the common-mode rejection ratio (CMRR) or converted to decibels (dB). The resistor matching of the discrete solution is not as good as the laser-trimmed resistor matching in the integrated solution, which can be seen from the output voltage vs. CMV curve in Figure 4. Figure 4.


Output Voltage vs. Common-Mode Voltage - AD8271 vs. Discrete Solution

Assuming an ideal op amp, the CMRR is:



where Ad is the gain of the difference amplifier and t is the resistor tolerance. Therefore, for unity gain and 1% resistors, the CMRR is 50 V/V or about 34 dB; when using 0.1% resistors, the CMRR increases to 54 dB. Even with an ideal op amp with infinite common-mode rejection, the overall CMRR is limited by the resistor matching. Some low-cost op amps have a minimum CMRR of 60 dB to 70 dB, making the error even worse.

Low Tolerance Resistors

Amplifiers generally perform well over their specified operating temperature range, but the temperature coefficients of external discrete resistors must be considered. For amplifiers with integrated resistors, the resistors can be drift trimmed and matched. Layout usually places the resistors close together so they drift together, reducing their offset temperature coefficient. In the discrete case, the resistors are spread out on the PCB and are not as well matched as the integrated solution, resulting in a worse offset temperature coefficient, as shown in Figure 5.


Figure 5. System Offset vs. Temperature—AD8271 vs. Discrete Solution

Four-resistor difference amplifiers are widely used, both discrete and monolithic. Since only one device is placed on the PCB instead of multiple discrete components, boards can be built more quickly and efficiently, saving a lot of area.

Noise gain, input voltage range, and CMR (to 80 dB or more) should be carefully considered to achieve a stable and production-worthy design. The resistors are made of the same low-drift thin-film material, so they provide excellent ratio matching over temperature.

Conclusion

It is easy to see the difference between amplifiers with built-in gain-setting resistors and discrete difference amplifiers.