SBAA267B February   2018  – September 2024 ADS8912B , OPA197 , REF5045 , THS4551

 

  1.   1
  2.   2
  3.   Revision History
  4.   Trademarks

Input Voltage (OPA197 Buffers) THS4551 Output, ADC Input ADS8912B Digital Output
VinP = –12V, VinN = +12V, VinMin (Dif) = –24V VoutDif = –4.00V, VoutP = 0.25V, VoutN = 4.25V 238E3H -11650910
VinP = +12V, VinN = –12V, VinMax (Dif) = +24V VoutDif = +4.0V, VoutP = 4.25V, VoutN = 0.25V 1C71CH +11650810
Supplies and Reference
HVDD HVSS Vcc Vee Vref Vcm
+15V –15V +5.0V 0V +4.5V 2.5V

Design Description

This analog front end (AFE) and SAR ADC data acquisition solution can measure true differential voltage signals in the range of ±24V (or absolute input range VinP = ±12V, VinN = ±12V) offering high-input impedance supporting data rates up to 500ksps with 18-bit resolution. A precision, 36-V rail-to-rail amplifier with low-input bias current is used to buffer the inputs of a fully-differential amplifier (FDA). The FDA attenuates and shifts the signal to the differential voltage and common-mode voltage range of the SAR ADC. The values in the component selection section can be adjusted to allow for different input voltage levels.

This circuit implementation is used in accurate measurement of true differential voltage in parametric measurement units (PMUs), precision multifunction input and output DAQs, and analog input modules used in Programmable Automation Control (PAC), Discrete Control System (DCS), and Programmable Logic Control (PLC) applications.

Specifications
Specification Goal Calculated Simulated
Transient ADC Input Settling (500ksps) << 1 LSB; << 34µV N/A 0.5µV
Noise (at ADC Input) 10µVRMS 9.28µVRMS 9.76µVRMS
Bandwidth 1.25MHz 1.25MHz 1.1MHz

Design Notes

  1. Verify the linear range of the op amp (buffer) based on the common mode, output swing specification for linear operation. This is covered in the component selection section. Select an amplifier with low input bias current.
  2. Find ADC full-scale range and common-mode range specifications. This is covered in the component selection.
  3. Determine the required attenuation for the FDA based on the input signal amplitude, the ADC full-scale range and the output swing specifications of the FDA. This is covered in the component selection section.
  4. Select COG capacitors to minimize distortion.
  5. Use 0.1% 20ppm/°C film resistors or better for good accuracy, low gain drift, and to minimize distortion.
  6. Understanding and Calibrating the Offset and Gain for ADC Systems covers methods for error analysis. Review the link for methods to minimize gain, offset, drift, and noise errors
  7. Introduction to SAR ADC Front-End Component Selection covers methods for selecting the charge bucket circuit Rfilt and Cfilt. These component values are dependent on the amplifier bandwidth, data converter sampling rate, and data converter design. The values shown here will give good settling and AC performance for the amplifier, gain settings, and data converter in this example. If the design is modified, a different RC filter must be selected. Refer to the Precision Labs videos for an explanation of how to select the RC filter for best settling and AC performance.

Component Selection and Settings for Buffer Amplifier and FDA

  1. Verify the buffer amplifier input range for linear operation:
  2. Verify the buffer amplifier output range for linear operation:
  3. Find ADC full-scale input range. In this circuit, VREF = 4.5V:
  4. Find the required ADC common-mode voltage:
  5. Find FDA absolute output voltage range for linear operation:
  6. Find FDA differential output voltage range for linear operation. The general output voltage equations for this circuit follow:
  7. Find the FDA differential input voltage range:
  8. Find FDA required attenuation ratio:
  9. Find standard resistor values to set the attenuation:
  10. Find Cƒ for cutoff frequency ƒc, RfINA = 1kΩ:

DC Transfer Characteristics

The following graph shows a linear output response for differential inputs from +24V to –24V.

AC Transfer Characteristics

The simulated bandwidth is approximately 1.1MHz and the gain is –15.62dB which is a linear gain of approximately 0.166V/V (attenuation ratio 6.04V/V).

Noise Simulation

Note that calculated and simulated match well. Refer to Calculating the Total Noise for ADC Systems for detailed theory on this subject.

Stability Simulation

The following circuit is used in TINA to measure loop gain and verify phase margin using AC transfer analysis in TINA. Resistors RISO = 10Ω are used inside the feedback loop to increase phase margin. The circuit has 45 degrees of phase margin. Refer to TI Precision Labs - Op Amps: Stability 4 for detailed theory on this subject.

Transient ADC Input Settling Simulation

The following simulation shows settling to a 24-V DC differential input signal with the OPA197 buffers inputs set at +12V and –12V. This type of simulation shows that the sample and hold kickback circuit is properly selected. Refer to Refine the Rfilt and Cfilt Values for detailed theory on this subject.

Design Featured Devices

Device Key Features Link Similar Devices
ADS8912B(1) 18-bit resolution, 500-ksps sample rate, integrated reference buffer, fully-differential input, Vref input range 2.5V to 5V. 18-Bit, 1MSPS, 1-Ch SAR ADC with Internal VREF Buffer, Internal LDO and Enhanced SPI Interface Analog-to-digital converters (ADCs)
THS4551 FDA, 150-MHz bandwidth, Rail-to-Rail Output, VosDriftMax = 1.8µV/°C, en = 3.3nV/rtHz Low Noise, Precision, 150MHz, Fully Differential Amplifier Operational amplifiers (op amps)
OPA197 36V, 10-MHz bandwidth, Rail-to-Rail Input/Output, VosMax = ±250µV, VosDriftMax = ±2.5µV/°C, bias current = ±5pA Single, 36V, precision, rail-to-rail input output, low offset voltage op amp Operational amplifiers (op amps)
REF5045 VREF = 4.5V, 3 ppm/°C drift, 0.05% initial accuracy, 4µVpp/V noise 4.5V, 3-µVpp/V noise, 3-ppm/°C drift precision series voltage reference Series voltage references
(1) The REF5045 can be directly connected to the ADS8912B without any buffer because the ADS8912B has a built in internal reference buffer. Also, the REF5045 has the required low noise and drift for precision SAR applications. The THS4551 provides the attenuation and common-mode level shifting to the voltage range of the SAR ADC. In addition, this FDA is commonly used in high-speed precision fully-differential SAR applications as it has sufficient bandwidth to settle to charge kickback transients from the ADC input sampling. The OPA197 is a 36-V operational amplifier that provides a very high input impedance front end, buffering the FDA inputs

Link to Key Files

Texas Instruments, SBAC183 source files, software support