Input | ADC Input | Digital Output ADS8568 |
---|---|---|
VinDiffMin = –8mV | CH_x = –10V | 8000H |
VinDiffMax = +22mV | CH_x = +10V | 7FFFH |
AVDD | Vee | Vdd | |
---|---|---|---|
5.0V | 3.3V | +15V | –15V |
Instrumentation amplifiers are optimized for low noise, low offset, low drift, high CMRR and high accuracy but these instrument amplifiers are not able to drive a precision ADC to settle the signal properly during the acquisition time of ADC. This design shows an example of how to set the gain and offset shift to amplify a low level asymmetric input signal. Also, the high gain limits the INA828 instrumentation amplifier bandwidth, so an OPA827 op amp is used as a buffer so that the ADS8568 full sampling rate can be achieved. A related cookbook circuit shows a simplified approach that does not include the wide bandwidth buffer (Driving High Voltage SAR ADC with an Instrumentation Amplifier), this simplified approach has limited sampling rate as compared to the buffered design in this document. Also Driving High Voltage SAR ADC with a Buffered Instrumentation Amplifier, analyzes this design in unity gain. This circuit implementation is applicable to all Bridge Transducers in PLC’s and Analog Input Modules that require precision signal-processing and data-conversion.
Specification | Goal | Calculated | Simulated |
---|---|---|---|
Transient Settling Error | >0.5 LSB (152µV) | NA | 0.36µV |
Noise | 1.1mV | 1.14mV | |
System Offset Error | 33.6mV | NA | |
System Offset Drift | 334µV/°C | NA | |
System Gain Error | 0.53% | NA | |
System Gain Drift | 54.2ppm/°C | NA |
The following graph shows a linear output response for inputs from differential –10V to +10V. See Determining a SAR ADC’s Linear Range when using Instrumentation Amplifiers for detailed theory on this subject. The full-scale range (FSR) of the ADC falls within the linear range of the Instrumentation Amplifier.
The bandwidth for this design is simulated to be 14.8kHz and the gain is 56.4dB (667.7V/V). The bandwidth limit is set by a combination RC filter (fcRC = 15.9kHz) and the instrumentation amplifier (fcINA=49.2kHz).
The OPA827 buffer (22-MHz GBW) is used because it is capable of responding to the rapid transients from the charge kickback of the ADS8568. The op-amp buffer allows the system to achieve the ADS8568 maximum sampling rate of 510kSPS. The following simulation shows settling to a full-scale DC input signal with the INA828 and OPA827 buffer, and ADS8568. This type of simulation shows that the sample and hold kickback circuit is properly selected to meet desired ½ of a LSB (152µV). See Introduction to SAR ADC Front-End Component Selection for detailed theory on this subject.
The section walks through a simplified noise calculation for a rough estimate. We neglect the noise from the OPA827 as the noise of the INA828 is dominant also neglect resistor noise in this calculation as it is attenuated for frequencies greater than 15.92kHz.
Note that calculated and simulated match well (simulated = 563µVRMS, calculated = 595µVRMS). See TI Precision Labs - Op Amps: Noise 4 for detailed theory on amplifier noise calculations and Calculating Total Noise for ADC Systems for data converter noise.
The following offset and offset drift calculations are dominated by the instrumentation amplifier since it is in high gain. Gain error calculations include the gain error of the ADC and instrumentation amplifier. For offset and gain error, the maximum room temperature value is used. See Statistics Behind Error Analysis for details on system gain and offset error.
Device | Key Features | Link | Other Possible Devices |
---|---|---|---|
ADS8568 | 16-bit resolution, SPI, 500kSPS sample rate, single-ended input, simultaneous sampling, internal reference, programmable range up to ±12V. | 16-bit, 8-channel, simultaneous-sampling, bipolar-input, SAR analog-to-digital converter (ADC) | Analog-to-digital converters (ADCs) |
INA828 | Bandwidth 1MHz (G=1), low noise 18nV/√ Hz, low offset ±40μV, low offset drift ±0.4μV/°C, low gain drift 0.1ppm/°C (Typical values) | 50-μV Offset, 7-nV/√Hz Noise, Low-Power, Precision Instrumentation Amplifier | Instrumentation amplifiers |
OPA827 | Gain bandwidth 22MHz, low noise 4nV/√ Hz, low offset ±75μV, low offset drift ±0.1μV/°C (Typical values) | Low-noise, high-precision, JFET-input operational amplifier | Operational amplifiers (op amps) |
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