AC-couple analog signals without a dc bias source

-February 03, 2000

A conventional ac-coupling circuit in a single-supply system comprises a series capacitor and a shunt resistor to ground. Unfortunately, the negative peaks of the input signal can exceed the -0.3V input operating-range limits of an ADC, such as the LTC1402 serial 12-bit sampling ADC. To avoid going below ground, the circuit must return the shunt input resistor to a midsupply voltage source. This problem is classic with all single-supply ADCs. You can use fully differential analog inputs, such as those of the LTC1402 2.2M-sample/sec, 12-bit, serial ADC, to ac-couple an analog signal without this midsupply bias voltage.

The ADC inputs derive the common-mode dc operating voltage directly from the input signal. The circuit has two requirements: The analog input signal must remain between 0V and the 5V supply voltage, and the ac transients must remain below the ±2V bipolar input range of the ADC. In Figure 1a, R1 and the grounded C1 at the AIN- input of the ADC cancel the low-frequency signals and provide the basic ac-coupling function. R2 and its shunt capacitor, C2, at the ADC's AIN+ input cancel the sampling current bias offset. The optional C3-R3 46-MHz lowpass network isolates the ADC input from sampling-glitch-sensitive circuitry.

The frequency response for the values in the circuit has a low-cutoff pole at 1 kHz and low-frequency stopband rejection in excess of -60 dB, as set by the common-mode-rejection specification of the ADC, independent of RC-component-match accuracy (Figure 1b). The LTC1402 accepts wide bandwidth, full-scale, 4V p-p signals as great as 80 MHz. This ac-coupling circuit adds no distortion to the input signal. You can couple a 1.1-MHz Nyquist frequency sine wave into the ADC while keeping the THD below -82 dB.(DI #2479)

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