Single-/Dual-Supply Precision Analog Limiter Handles Wideband Signals

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As the name implies, a limiter circuit (sometimes called a “clipper”) prevents a signal from exceeding desired thresholds, which would otherwise cause saturation and overload of the following stage. The circuit in Figure 1 precisely limits the wideband input signal V_i at the V_REF level and can be used with both single and dual power supplies.

Output signal V_o1 of the first stage, U1_1 (one-half of an LT1810, a dual 180-MHz, 350-V/µs low-distortion op amp), is precisely limited at the level of V_REF , but with inverse polarity compared with the input signal. U1_1 has a gain of ‒1 for the input signal V_i, and a gain of +2 for the reference signal V_REF.

Since the positive input of U1_1 is referenced to V_REF, the output signal is referenced to 2 × V_REF. Thus, while the input signal rises from zero to V_REF, the output signal of the first stage decreases from 2 × V_REF to V_REF. The second stage of limiter U1_2 is an inverting amplifier with the gain of ‒1. It’s referenced to the V_REF as well, and restores the original dc level and polarity of the input signal.

1. This analog circuit, based on the LT1810 op amp, precisely limits input signal V_i at reference level V_REF.

For the output of the first stage, V_o1 = 2 × V_REF  - V_i if V_i ≤ V_REF  and V_o1 = V_REF if V_i ≥ V_REF, while the output signal of the second stage is V_o2 = 2 × V_REF ‒ V_o1 = V_i, if V_i ≤ V_REF, and V_o2 = V_REF if V_i ≥ V_REF. (Note that the output-dc level, along with the limiting point, may be shifted by an optional network of R6 and R7; otherwise, that network isn’t used.)

D1 is a part of the negative feedback of U1_1, making it an "ideal diode"—as soon as V_i reaches the V_REF level, U1’s output voltage increases until it compensates for voltage drop V_d of the diode. When V_i is less than V_REF and D1 is turned off, diode D2 is on and sends the output current of U1 directly to its input. This prevents U1 from negative saturation, which would significantly decrease the switching speed of the first stage.

The voltage level on the output pin of U1_1 is (2 × V_REF) ‒ V_i + V_d, and it reaches its maximum level with the minimum level of the input signal. This determines the maximum possible level of V_REF, which doesn’t cause the distortion of the limiter's output signal at its minimum level of V_REF = [(V+) + V_i - V_d]/2. Here, V+ is the maximum output voltage of op amp at the chosen V_CC, which is almost equivalent to V_CC for the rail-to-rail op amp.

For the same reason, the minimum level of V_REF should not be lower than V_d; however, the input signal may significantly exceed V_REF.

2. The LTspice simulation shows the limiter's response for a 1-MHz sine-wave signal (a) and a 2-µs piecewise-linear signal (b).

The LTspice simulation shows the limiter's responses for a 1-MHz sine-wave signal with 8-V peak-to-peak amplitude (Fig. 2a), and for a 10-V, 2-µs piecewise linear (PWL) signal crossing the limitation level several times (Fig. 2b), both with V_REF of 5.5 V. The limiter is quite accurate within the entire output-voltage range of the op amp. The simulation demonstrates that limitation errors don’t exceed 8 mV and output setting time is within 30 ns around the limitation point. When compared to the "simple op amp clipper" of Ref. 1, this limiter works at much-higher frequencies and with significantly lower distortion.

References

1. Thomas Mosteller and Aaron Schultz, "Op Amps Make Precision Clipper, Protect ADC" Electronic Design, November 2016.

2. Datasheet, LT1810 - Dual 180MHz, 350V/µs Rail-to-Rail Input and Output Low Distortion Op Amps.

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