SBAA507 March 2021 ADS8578S , ADS8584S , ADS8586S , ADS8588H , ADS8588S , ADS8598H , ADS8598S , ADS8661 , ADS8664 , ADS8665 , ADS8668 , ADS8671 , ADS8674 , ADS8675 , ADS8678 , ADS8681 , ADS8684 , ADS8684A , ADS8685 , ADS8688 , ADS8688A , ADS8688AT , ADS8689 , ADS8691 , ADS8694 , ADS8695 , ADS8698 , ADS8699 , OPA2990 , OPA4990 , OPA990
Normal Bipolar Input Signal Range | Bipolar Input Range on ADS8681 |
Supply and Voltage Reference | ||||
---|---|---|---|---|---|---|
VG_Min | VG_Max | VADC_Min | VADC_Max |
AVDD |
DVDD |
VREF |
–12.88V |
+12.88V |
–12.88V |
+12.88V |
+5V |
+3.3V |
4.096V |
This circuit shows a solution to detect if the ADS8681 successive approximation register (SAR) analog-to-digital converter (ADC) input is floating. The ADS8681 device features a significant amount of signal chain integration, including high resistive input impedance, a programmable gain amplifier (PGA), and an ADC input driver. These features eliminate the requirement for driving the ADC inputs with a high-bandwidth amplifier. Therefore, a low-bandwidth, low-cost amplifier in a non-inverting unity gain configuration can be used in a solution for detecting a floating input. This solution has a minimal impact on the system performance metrics including signal-to-noise ratio (SNR) and total harmonic distortion (THD). This circuit is useful in end equipment such as: Analog input module, Servo drive functional safety module, Motor drives, and Factory automation and control.
Specification | Goal | Calculated and Simulated Value |
---|---|---|
Noise | < 10µV | Calculated: 4.3µV Simulated: 4.38µV |
Phase Margin | > 45° | 64° |
Input Signal Voltage (VG) | ADS8681 Input Range (VADC) | Expected Normal Voltage on ADC Input (VIN) | Expected Voltage on ADC Input for Float (VIN_T) | |
---|---|---|---|---|
Maximum | +12.88V | +12.88V | +11.88V | +12.38V |
Minimum | –12.88V | –12.88V | –11.88V | +12.38V |
R3 is selected as 1kΩ for a small gain and offset error. Therefore, R4 is determined by the following equation:
A 12-kΩ and 0.1% tolerance resistor is finally selected for R4. The resistor value can be adjusted according to the noise and margin required in the system.
R1 and R2 are selected to create the VT voltage during floating inputs. To achieve higher input impedance, R2 is selected as 10MΩ. R1 is determined by the following equation:
R1 is selected as 1.2MΩ which is a standard resistor value. Note that the tolerance on these resistors only impacts the float voltage accuracy.