How can we obtain the best SNR performance from an ADC?

There are several alternatives to reduce reference noise. Increasing the size of the bypass capacitor or using a simple low-pass RC filter on the reference output are not good alternatives. A large bypass capacitor on the reference output by itself does not produce a sufficiently low cutoff frequency to be effective. A passive RC filter by itself provides a low cutoff frequency, but produces an output voltage that varies with sampling frequency and temperature. Paralleling the outputs of multiple low-noise references is an effective alternative, but is expensive and consumes a lot of power. The reference filter presented here produces a low-noise reference voltage without significantly compromising the reference accuracy or temperature coefficient, and does so at only moderate power and cost.

The ADC used in this example is the LTC2508-32 (U1). The LTC2508-32 is a low noise, low power, 32-bit SAR ADC with a lowpass digital filter that has four pin-selectable downsampling factors (DF) ranging from 256 to 16384. A low noise, low temperature drift reference is necessary to realize the full performance of the LTC2508-32.

The reference used in this example is the LTC6655-5 (U2). The LTC6655-5 offers high accuracy (±0.025% maximum), exceptionally low noise (0.67ppm RMS typical) and drift (2ppm/°C maximum) performance. Even with its exceptionally low noise performance, the LTC6655-5 still degrades the SNR performance of the LTC2508-32.

The LTC2057 (U3) is a zero-drift op amp with suppressed 1/f noise. The LTC2057 has an input bias current (IB) of less than 200pA, a maximum offset voltage of 4μV, and a maximum offset voltage temperature coefficient of 0.015μV/°C. This is significantly lower than the temperature coefficient of the LTC6655-5 (2ppm/°C = 10μV/°C).

The LT6202 (U4) is a low noise, fast settling op amp with the necessary high short circuit current capability to drive the required 47μF bypass capacitor on the LTC2508-32 REF pin.

The circuit in Figure 1 uses R2 and C3 to form a 0.8Hz filter to filter the output of the reference (U2). Capacitor C3 should be a film capacitor. Tantalum and electrolytic capacitors have high leakage and will produce an offset across R2. Ceramic capacitors exhibit a microphonic effect, which can cause increased noise at low frequencies. The filtered output is buffered by the high impedance input of U3. U3's 200pA maximum IB produces a maximum voltage drop of only 2μV across R2. This combines with the offset voltage of the LTC2057 to produce a maximum error of 6μV, which is relatively insignificant compared to the 0.025% (1.25mV) maximum initial accuracy specification of the LTC6655-5. U3 and U4 form a composite amplifier that has the low offset, offset temperature coefficient and suppressed 1/f noise of the LTC2057, and the fast settling characteristics of the LT6202. The REF pin of U1 draws charge from C1, and the amount of charge drawn varies with the sampling rate and output code. U4 must replenish this charge to keep the REF pin voltage fixed. R5 is used to isolate U4 from C1 to improve the stability at the REF pin. Ceramic capacitors with higher voltage and temperature ratings and larger physical size have lower voltage coefficients, providing a higher effective capacitance. For this reason, C1 should be a 1210 size, 10V rated X7R type ceramic capacitor.

As shown in Table 1, the LTC2508-32 behaves almost like theory, with the dynamic range increasing by almost 6dB for every 4x increase in the downsampling factor when the ADC inputs are connected together and the REF pin is driven directly by the LTC6655-5. Table 1 also shows that when the ADC is driven to nearly full scale, the SNR is up to 7.8dB less than the DR (with the LTC6655-5 driving the ADC REF pin directly). This is due to the noise of the reference. Using the circuit shown in Figure 1 to drive the REF pin of the LTC2508-32 can achieve up to 6.1dB of SNR improvement, as shown in Table 1.

Chopper-stabilized op amps such as the LTC2057 often exhibit tones at the chopping frequency and its odd harmonics. The LTC2057 employs circuitry to suppress these artifacts well below the offset voltage. This circuitry works in combination with the ADC’s ​​own filters to remove any visible tones from the op amp’s chopping frequency, as shown in the noise floor plot of Figure 2. The plot shown in Figure 2 is an average of five data captures in order to smooth out the noise floor to reveal even the smallest trace of any spurious tones.