Design Note 541: Introduction
The op amps used to drive 18-bit analog-to-digital converters (ADCs) typically draw as much supply current as the ADC itself, often with a maximum offset spec that is well above that of the ADC. If multiple ADC channels are required, the power dissipation from these drivers quickly rises to unacceptable levels.
If 18-bit precision is required (SNR, THD, VOS), but not high sampling rates, and the input signals are low frequency or DC, the simple buffer presented is capable of driving the LTC2348-18 8-channel simultaneous sampling ADC. It also achieves performance equivalent to typical specs for SNR, THD and offset performance with very low power dissipation.
The LTC2348-18 is a low noise, 8-channel simultaneous sampling 18-bit successive approximation register (SAR) ADC with wide input common mode range. With a ±10.24V input range, the LTC2348-18 achieves –109dB THD (typical), 96.7dB SNR (typical) with an offset of ±550μV (maximum) while dissipating only 140mW (typical) at 200ksps. When operated at the 10ksps rate of this application, the ADC’s power consumption drops to 45mW (typical) by using the device’s NAP mode.
The LT6020 is a dual micropower, 5V/μs precision rail-to-rail output op amp with input offset voltage of less than 30μV (maximum) that draws only 100μA per amplifier (maximum).
The circuit of Figure 1 shows the LT6020 op amp configured as a noninverting buffer driving the analog inputs of the LTC2348-18. Maximum power dissipation of each op amp is only 3mW. For all eight channels this adds up to only 24mW, approximately half the ADC power consumption at 10ksps.
The RC filter at the buffer output minimizes the noise contribution of the LT6020 and reduces the effect of the sampling transient caused by the MUX and the input sampling capacitor. For a chosen RC time constant, the R value should be kept as small as possible to reduce the voltage drop across the resistor. This results in a gain error if the filter output is not allowed to settle completely. The R value must be large enough to prevent excessive ringing at the op amp output, which adds to settling time and increases distortion.
The LTC2348-18 allows two different modes of operation. The first is a fully differential mode, which requires both analog inputs of each channel to be driven by a separate amplifier. The second is a pseudo-differential mode, which drives only a single analog input while grounding the other input. This second mode is used by the circuit of Figure 1. Using pseudo-differential drive means fewer components are required, as well as lower power dissipation. The disadvantage of using this mode is that the INL is slightly degraded.
Figure 2 shows an 8192-point FFT of the LTC2348-18 driven pseudo-differentially by the buffer of Figure 1. THD is –108dB and SNR is 95.8dBFS at 10ksps, which compares well with the typical specs of the LTC2348-18.
Figure 3 shows SNR and THD vs sampling rate. SNR stays fairly flat near 96dBFS up to 10ksps. THD starts to rise above –108dB at 10ksps.
Figure 4 shows SNR and THD vs input frequency. Both SNR and THD slowly degrade from the typical specs of the LTC2348-18 above 100Hz until at 1kHz SNR is 94dBFS and THD is –85dB.
Figure 5 shows the combined offset error of the LT6020 driver and ADC vs sampling rate. Offset is initially less than 1LSB and starts to degrade as the sampling rate exceeds 10ksps.
A simple driver for the LTC2348-18 18-bit, 200ksps, 8-channel simultaneous sampling SAR ADC—consisting of the LT6020 low power precision dual op amp configured as noninverting buffers—dissipates only 3mW per op amp (maximum), and at 10ksps the LTC2348-18 dissipates only 45mW. At a sampling rate of 10ksps, SNR is measured at 95.8dB, THD –109dB and offset is measured at less than 1LSB.