Input | ADC Input | Digital Output ADS8910 | ||||
---|---|---|---|---|---|---|
VinMin = –20V | VoutDif = 4.8V, VoutP = 4.9V, VoutN = 0.1V | 1EB85H or 12582910 | ||||
VinMax = 20V | VoutDif = –4.8V, VoutP = 0.1V, VoutN = 4.9V | 2147BH or –12582910 |
Vcc | Vee | Vref | Vcm |
---|---|---|---|
5.3V | 0V | 5V | 2.5V |
This design translates an input bipolar signal of ±20V into a fully differential ADC differential input scale of ±4.8V, which is within the output linear operation of amplifiers. The values in the component selection section can be adjusted to allow for different input voltage levels.
This circuit implementation is applicable in accurate voltage measurement applications such as Battery Maintenance Systems, Battery Analyzers, battery cell formation and test equipment, ATE, and Remote Radio Units (RRU) in wireless base stations.
Specification | Calculated | Simulated | Measured |
---|---|---|---|
Transient ADC Input Settling | < 0.5LSB or 19µV | 6.6µV | N/A |
Noise | 20.7µVrms | 20.65µVrms | 30.8µVrms |
Bandwidth | 10.2kHz | 10.4kHz | 10.4kHz |
The following graph shows a linear output response for inputs from –20V to +20V. Refer to Determining a SAR ADC’s linear range when using operational amplifiers for detailed theory on this subject.
The bandwidth is simulated to be 10.4 kHz, and the gain is –12.4dB which is a linear gain of 0.12. See Op Amps: Bandwidth 1 for more details on this subject.
The following simulation shows settling to a –20V dc input signal. This type of simulation shows that the sample and hold kickback circuit is properly selected. Refer to Introduction to SAR ADC Front-End Component Selection for detailed theory on this subject.
The following simplified noise calculation is provided for a rough estimate. We neglect resistor noise in this calculation as it is attenuated for frequencies greater than 10kHz.
Note that calculated and simulated match well. Refer to Calculating the Total Noise for ADC Systems for detailed theory on this subject.
This performance was measured on a modified version of the ADS8910BEVM. The AC performance indicates SNR = 99.4dB, and THD = –116.4dB. See Introduction to Frequency Domain for more details on this subject.
The following measured result is for both inputs connected to ground. The histogram shows the system offset and noise. The standard deviation in codes is given by the EVM GUI (0.81), and this can be used to calculate the RMS noise (30.9µV rms) as shown in the following equation.
Device | Key Features | Link | Similar Devices |
---|---|---|---|
ADS8900B(1) | 18-bit resolution, 1-Msps sample rate, Integrated reference buffer, fully differential input, Vref input range 2.5V to 5V. | 20-bit, 1MSPS, one-channel SAR ADC with internal VREF buffer, internal LDO and enhanced SPI | Precision ADCs |
OPA320(2) | 20-MHz bandwidth, Rail-to-Rail with Zero Crossover Distortion, VosMax = 150µV, VosDriftMax = 5uV/°C, en = 7nV/rtHz | Precision, zero-crossover, 20MHz, 0.9pA Ib, RRIO, CMOS operational amplifier | Precision op amps (Vos<1mV) |
REF5050(3) | 3ppm/°C drift, 0.05% initial accuracy, 4µVpp/V noise | 5V, 3µVpp/V noise, 3ppm/°C drift precision series voltage reference | Voltage references |
Texas Instruments, SBAC171 design files, software support
Changes from Revision A (January 2019) to Revision B (September 2024)
Changes from Revision * (December 2017) to Revision A (January 2019)
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