“Some ideal op amp configurations will assume that the feedback resistors are perfectly matched. In fact, the non-ideality of the resistor will affect various circuit parameters, such as common mode rejection ratio (CMRR), harmonic distortion and stability. As shown in the example in Figure 1, configuring a single-ended amplifier to level-shift the ground reference signal to a common-mode voltage of 2.5V requires a good CMRR. Assuming that the CMRR is 34dB and there is no input signal, the 2.5V level shifter will produce an output offset of 50mV, which may even overwhelm the 12-bit analog-to-digital converter (ADC), the least significant bit (LSB) and offset of the driver. Shift error.
Some ideal op amp configurations will assume that the feedback resistors are perfectly matched. In fact, the non-ideality of the resistor will affect various circuit parameters, such as common mode rejection ratio (CMRR), harmonic distortion and stability. As shown in the example in Figure 1, configuring a single-ended amplifier to level-shift the ground reference signal to a common-mode voltage of 2.5V requires a good CMRR. Assuming that the CMRR is 34dB and there is no input signal, the 2.5V level shifter will produce an output offset of 50mV, which may even overwhelm the 12-bit analog-to-digital converter (ADC), the least significant bit (LSB) and offset of the driver. Shift error.
For operational amplifiers, 34dB is a less than ideal CMRR. However, regardless of the performance of the operational amplifier, a feedback network composed of 1% tolerance resistors will limit the CMRR to 34dB. Highly matched resistors (such as the resistors with matching accuracy of 0.01%, 0.025% and 0.05% provided by the LT5400) ensure that designers can approach or meet the performance indicators declared in the amplifier product manual. This design point compares the LT5400 with thick film, 0402, 1% tolerance surface mount resistors, and studies the CMRR, Harmonic distortion and stability.
Common mode rejection ratio
In order to obtain accurate measurement results in the presence of common mode noise, it is important to have a high CMRR. The input CMRR is defined as the ratio of the differential gain (VOUT(DIFF)/VIN(DIFF)) to the input common-mode to differential conversion gain (VOUT(DIFF)/VIN(CM)). In an ideal single-ended and fully differential amplifier, only the input differential level will affect the output voltage. However, in actual circuits, the available CMRR of resistor mismatches poses a limitation. Let’s study this circuit configured to attenuate a ±10V signal to a ±2V signal. When a typical surface mount resistor with a matching accuracy of 2% (1% tolerance) is used, the worst-case CMRR generated from the resistor is 30dB. When a resistor with a tolerance of 0.01% (0.02% matching accuracy) is used, the worst-case CMRR generated by the resistor is 70dB. One of the limiting factors in the CMRR formula is:
This expression is simplified to the resistance matching ratio of a typical resistor, but the LT5400 goes one step further, improving CMRR by limiting the matching between resistor pairs R1/R2 and R4/R3. By defining this formula as the CMRR matching formula, the accuracy provided by the LT5400 is better than when only the resistor matching ratio is used.For example, the LT5400A can ensure
Thus, the worst-case CMRR is increased to 82dB.
The CMRR produced by this circuit in the experimental test is 50.7dB (to a large extent limited by the matching accuracy of the resistors, using 1% tolerance resistors) and 86.6dB (using LT5400). In this case, a 2.5V common-mode input will produce 1.5mV (using 1% thick film resistors) and 23μV (using LT5400) offsets, making it suitable for 18-bit ADC applications where DC accuracy is very important.
Harmonic distortion is also important when selecting resistors for precision applications. Depending on the size and material, a large signal voltage across the resistor may cause a significant change in resistance. This problem occurs in many chip resistors, and as the power level on the resistor increases, this situation will become more serious. Table 1 compares the distortion performance indicators of thick film, through-hole and LT5400 resistors based on high-power drives and similar power drives. The comparison result shows: For a given signal, compared with other resistor types, the signal distortion caused by the LT5400 is much smaller.
Figure 3 shows the distributed capacitance model between resistors in the LT5400. In order to achieve high-precision matching and tracking in the LT5400, many small silicon chromium (SiCr) resistors are configured in series and parallel. Due to the complex interleaving combination, the LT5400 resistor can be simulated as a series of infinitesimal resistors with parasitic capacitance between adjacent sections and between each section and the exposed pad. In contrast, standard surface-mount resistors that do not use this tight layout exhibit much smaller parasitic capacitance.
When the exposed pad is grounded, the influence of the capacitance between the resistors can be reduced. However, even after the exposed pad is grounded, the capacitor still affects the stability of the circuit by forming a parasitic pole (about the product of the total resistance and the total capacitance).
Since the overshoot is inversely proportional to the phase margin, minimizing the step response overshoot is a good way to ensure circuit stability. The uncompensated LT5400 configuration produces an overshoot of 27%, while the 0402 configuration produces an overshoot of 17%. However, the compensation capacitor required to achieve 8% overshoot is roughly the same in both configurations: LT5400 is 18pF and 0402 resistor is 15pF. When the compensation capacitors used are almost the same, the stability characteristics of the two circuits are quite similar.
Since product manual specifications assume ideal components, the actual performance of high-precision amplifiers and ADCs is often difficult to achieve. The precisely matched resistor network (such as the resistor network provided by the LT5400) can achieve precise matching that is several orders of magnitude higher than that of discrete components, ensuring that the performance indicators declared in the high-precision IC product manual are achieved.