Guy Hoover, Linear Technology
Design Note 541
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 LTC®2348-18 8-channel simultaneous sampling ADC. It also achieves performance equivalent to typical specs for SNR, THD and offset performance with very low power dissipation.
Circuit Description
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 ±10.24 V input range, the LTC2348-18 achieves –109 dB THD (typical), 96.7 dB SNR (typical) with an offset of ±550 μV (maximum) while dissipating only 140 mW (typical) at 200 ksps. When operated at the 10 ksps rate of this application, the ADC’s power consumption drops to 45 mW (typical) by using the device’s NAP mode.
The LT6020 is a dual micropower, 5 V/μs precision rail-to-rail output op amp with input offset voltage of less than 30 μV (maximum) that draws only 100 μA per amplifier (maximum).
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| Figure 1. | LT6020 Buffer Driving the LTC2348-18 8-Channel Simultaneous Sampling SAR ADC. |
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 3 mW. For all eight channels this adds up to only 24 mW, approximately half the ADC power consumption at 10 ksps.
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.
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| Figure 2. | 8192-Point FFT for the Circuit of Figure 1. |
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.
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| Figure 3. | SNR and THD vs Sampling Rate for the Circuit of Figure 1. |
Circuit Performance
Figure 2 shows an 8192-point FFT of the LTC2348-18 driven pseudo-differentially by the buffer of Figure 1. THD is –108 dB and SNR is 95.8 dBFS at 10 ksps, 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 96 dBFS up to 10 ksps. THD starts to rise above –108 dB at 10 ksps.
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| Figure 4. | SNR and THD vs Input Frequency for the Circuit of Figure 1. |
Figure 4 shows SNR and THD vs input frequency. Both SNR and THD slowly degrade from the typical specs of the LTC2348-18 above 100 Hz until at 1 kHz SNR is 94 dBFS and THD is –85 dB. Figure 5 shows the combined offset error of the LT6020 driver and ADC vs sampling rate. Offset is initially less than 1 LSB and starts to degrade as the sampling rate exceeds 10 ksps.
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| Figure 5. | Combined ADC and Driver Offset vs Sampling Rate for the Circuit of Figure 1. |
Conclusion
A simple driver for the LTC2348-18 18-bit, 200 ksps, 8-channel simultaneous sampling SAR ADC – consisting of the LT6020 low power precision dual op amp configured as noninverting buffers – dissipates only 3 mW per op amp (maximum), and at 10 ksps the LTC2348-18 dissipates only 45 mW. At a sampling rate of 10 ksps, SNR is measured at 95.8 dB, THD –109 dB and offset is measured at less than 1 LSB.
