Data acquisition
An important advantage of the input high-Z implemented in the AD4696 family is that the circuits that perform the high-Z function can all be power cycled at the rate of the conversion. So, the power consumption of the high-Z function would scale linearly with the throughput of the ADC, just like the core SAR ADC itself. This offers remarkable flexibility compared to the traditional, rather rigid signal chain designs.
The input high-Z function is also built into
the LTspice model of the AD4696. The first and second charge kicks are accurately modelled, enabling reliable simulation of the settling artefacts for signal chain designs.
SOME SUBTLETIES
Recall that the NP0 capacitor also provided wideband noise filtering of the signal chain. Now that we want to eliminate the capacitor, we must find other ways to filter the noise. A simple way to achieve the same effective signal chain noise bandwidth is to increase the external series resistance. The AD4696 has a 60pF internal capacitor in series with a 240Ω typical internal resistor. By setting the external resistor, we can tune the signal chain noise bandwidth to a desired value. Without the NP0 capacitor, the external resistance plays an important role in the noise performance, linearity, and accuracy of the signal chain. A small value resistor can help settle sampling charge kicks quickly and thus improve linearity and accuracy, but it comes at the cost of increased overall noise due to higher effective noise bandwidth. Conversely, a large value resistor filters noise better, but it comes at the expense of degraded linearity and accuracy. As described in the next section, a big advantage of the high-Z technology in AD4696 is that it allows large value resistors to be used (for better noise filtering) without degrading the linearity and accuracy. This allows optimisation for all parameters in the signal chain - noise, linearity, accuracy, power, and solution size.
MEASUREMENT RESULTS Measurements have been performed with a 2kΩ external resistor and without any NP0 capacitor. The results show a massive improvement in AC and DC performance with the analogue input high-Z enabled. The experiment involves running the core ADC of AD4696 at 1MSPS but choosing an increasing number of channels as part of a round-robin sequence. Data are collected on one channel, while the other channels in the sequence are provided with 0V inputs. Figure 4 shows the distortion performance of the channel of interest for a 1kHz, –1dBFS tone. As the channels are sequenced with the high-Z disabled, nonlinear settling errors occur because the sampling capacitor is not charged to the subsequent channel voltage. This results in significant distortion. With the high-Z enabled, there is a massive
62
Figure 4. THD vs. number of channels in a sequence. Test tone: 1kHz, –1dBFS.
Figure 5. DC settling errors in LSBs at 16-bit level.
improvement in the distortion performance. Figure 5 shows the DC steady-state settling
errors with and without the high-Z function. In this test, the channel of interest is provided with a near full-scale input and the other channels in the sequence are driven with 0V. Conversions are performed on the channel of interest while adding an increasing number of channels to the sequence, and the shift in the average output code from the expected code is plotted. When operating the core ADC at lower throughputs than 1MSPS, the user may need to further lower the effective signal chain noise bandwidth to limit the analogue front- end noise aliasing. This would necessitate higher resistance values, and the high-Z function helps immensely with maintaining the performance under these conditions.
CONCLUSION
The input high-Z technology implemented in the AD4696 family of parts offers unparalleled benefits for muxed SAR applications such as reduced system level power, reduced footprint, and reduced component count, to name a few, while maintaining high levels of AC performance and DC accuracy. It eliminates the need for a dedicated driver amplifier and a kickback absorption capacitor per channel. The power consumption for the high-Z function itself scales with the throughput of the ADC, offering remarkable flexibility and versatility for system-level design. An LTspice model of the AD4696 is available to simulate the effect of charge kicks in any system the user wishes to design.
Analog Devices
www.analog.com September 2022 Instrumentation Monthly
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82
