Engineers developing multichannel sensor solutions are often forced to develop custom solutions that may not be scalable across different use cases. This can lead to inflated R&D schedules and costs. Analog Devices’ Easy Drive SAR ADCs use innovative input architectures to simplify analog front end (AFE) design. Simplifying the AFE can lead to more flexible, scalable solutions that can address multiple use cases.
A conventional multi-channel sensor signal chain construction is shown below. It typically uses discrete components for the AFE to customize each sensor channel. The ADCs may either be discrete or a multichannel package.
Figure 1. Traditional multi-channel sensor signal chain.
Many sensors have high source impedance, requiring a high impedance input stage to avoid loading the sensor. Some high impedance sensor examples include ECG/EEG electrodes, piezoelectric-based sensors, pH or ion-selective electrodes, and capacitive sensors. Many of these sensors have bipolar outputs, requiring a bipolar input stage. Finally, as the type of sensor varies, the output may range from a few millivolts up to + 20 volts, requiring a dynamic range of greater than 80 dB for the total system.
One of the several innovations employed by ADI’s Easy Drive ADCs is to incorporate a high impedance input that can practically eliminate the need for an external AFE in the signal chain. The AD485x family of 20-bit and 16-bit, 8 or 4 channel Easy Drive ADCs are solutions that include a differential, high impedance buffer (1000 GΩ) at each channel input and have flexible input power supply rails that can exceed + 40 volts, with a maximum span up to 48 volts. Finally, their Seamless High Dynamic Range (SHDR) mode automatically adjusts the internal gain for each input sample to maximize the signal to noise ratio (SNR), effectively optimizing the dynamic range on a sample-by-sample basis. Each channel can be independently configured through software to adapt to the full scale output of an attached sensor. Furthermore, the ADC automatically adjusts the output code, normalizing each channel sample value to the full scale input range of the channel. The following plot illustrates the difference in noise performance for different input ranges when SHDR is enabled and disabled.
Figure 2. AD485x Seamless High Dynamic Range noise performance
The AD485x Easy Drive ADC family also offers per-channel digital gain and offset correction, plus digital averaging to improve noise performance. Each channel has a dedicated ADC core supporting sample rates up to 1 MSPS. With an embedded voltage reference (4.096V) and internal power supply bypassing, the AD485x family is a multi-channel data acquisition system (DAS) on a chip. Using an AD485x device, the system of Figure 1 can be reduced to the simplified solution shown below:
Figure 3. AD485x Data Acquisition System solution
Achieving all the capabilities offered by the AD485x family through conventional signal chain design would require an extremely complicated circuit, more components, and most likely, a high power host processor and complex software. This increased complexity leads to higher R&D costs and extended development time. The family’s 7 mm x 7 mm BGA package means that a single, complete signal chain can occupy as little as 6.125 mm2 equivalent per channel, or the equivalent of 2.5 mm x 2.5 mm per channel.
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AD4858: Buffered, 8-Channel Simultaneous Sampling, 20-Bit 1 MSPS DAS. The AD485x family offers 4 or 8 channels, simultaneously sampling, and a complete data acquisition system on-a-chip, with per-channel programmable input range and seamless high dynamic range to maximize SNR.
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Explore the AD485x family of Easy Drive ADCs from the Analog Devices Product Selection Table.
To learn more about Easy Drive circuit design techniques, see the following references.
- Tech Note: Can Analog Devices' Easy Drive ADCs simplify multi-channel sensor signal chain design?
- Video: Analog Devices EasyDrive︎ ADCs
- Blog: LTspice®︎ and EasyDrive︎ — A One-Two-Punch for Design Efficiency
- KWIK Note: SAR ADC Kickback Filter Design
- Tech Note: Can Analog Devices’ Easy Drive ADCs save cost and board space?