SBAA376A june 2019 – may 2023 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
|Input||ADC Input||Digital Output ADS8588S|
|VinMax = +10 V||CH_nP = +10 V||7FFFFH|
|VIN = 0 V||CH_nP = 0 V||0000H|
|VinMin = –10 V||CH_nP = –10 V||8000H|
|Power Supplies and Input|
|Absolute Maximum Input Voltage Rating(Vin_Abs)||Absolute Maximum Input Current Rating(Iin_Abs)|
|+5 V||+3.3 V||±10 V||±15 V||±10 mA|
This circuit describes a solution to protect high-voltage SAR ADCs with integrated analog front end (AFE) from electrical overstress. The protection is implemented with an external transient voltage suppressor (TVS) diode, current-limiting resistors, and an RC filter. This document shows the impact that the external protection clamp has on system performance and introduces how to improve the performance. This circuit is useful in the following end equipment: Multifunction relays, AC analog input modules, and Train control and management systems. For protecting low-voltage SAR ADC from electrical overstress, see Circuit for protecting low-voltage SAR ADC from electrical overstress with minimal impact on performance.
|Maximum continuous fault voltage||±40 V||No damage with 40-V peak sinusoidal wave applied|
|SNR (ADS8588S)||91.0 dB (±10-V range)||92.0 dB|
|THD (ADS8588S)||–114 dB||–81.8 dB|
Based on these guidelines, the PGSMAJ10CA bidirectional TVS diode from Taiwan Semiconductor Manufacturing Company, Ltd. ( TSMC®) is used in this design to protect the ADS8588S device which is a widely-used TVS diode. The specifications for three different TVS diodes are given later in this document to compare all the pertinent parameters.
The following equation shows the maximum current in RP during an electrical overstress fault event:
Using the maximum current (IMax) and maximum clamping voltage (VC_Max) on the PGSMAJ10CA TVS diode, calculate the maximum power dissipated on the diode during an electrical overstress fault event. The objective of this equation is to make sure that the correct power rating of TVS diode is used. For the PGSMAJ10CA diode, the steady-state power rating is 1W. Thus, there is margin in this design as we do not expect more then 491mW of continuous power.
The equation calculated Rfilt to be 200Ω. In this design, the 200Ω is rounded up to 1kΩ to add significant design margin. Rfilt can be adjusted to set the cutoff frequency of the filter as step 5 shows.
The following figure shows the ADC input voltage when a high-voltage continuous overvoltage sinusoidal wave signal (60 Vpp) is applied. Note that the external bidirectional TVS diode is turned on and the overvoltage sinusoidal wave signal has been clamped to ±12.5Vpeak, which is less than ±15-V absolute maximum input voltage (Vin_Abs) on the ADS8588S device, so the ADC device is protected from the overvoltage signal.
The following spectral analysis is measured using the ADS8588SEVM-PDK and overvoltage protection (OVP) board specifically designed for this test. The measured THD performance with all the protection circuitry including the TVS diode is worse than the typical specification in the ADS8588S 16-Bit, High-Speed, 8-Channel, Simultaneous-Sampling ADC With Bipolar Inputs on a Single Supply Data Sheet (measured SNR = 92dB,THD = –81.8dB).
The previous test result shows that the measured SNR performance meets the specification in the ADS8588S data sheet; however, the measured THD performance is worse than the specification in the data sheet. The key reason to cause the worse THD performance is the large capacitance variation which is changed with the voltage applied on the TVS diode, see the Electrical Overstress (EOS) and Electrostatic Discharge (ESD) on Analog-to-Digital Converters (ADC) video (part of the TI Precision Labs video series) for detailed theory and analysis on this subject.
There are two solutions to improve the system THD performance: use a low-capacitance variation bidirectional TVS diode, reduce input signal frequency with less impact from the capacitance and variation on the TVS diode, if this is acceptable in the system.
The capacitance of the TVS diode has a strong impact on distortion, and the following table compares the measured results for TVS diodes with different capacitance specifications. This table shows improved THD performance with decreasing capacitance on the TVS diodes. The low-capacitance bidirectional TVS diode, PGSMAJ10CA, from TSMC shows a better THD performance (–81.8dB) with the same protection circuitry. To achieve a better THD performance, use the TVS diode with further lower capacitance and variation.
MAX (IR at VR)
|Peak Pulse Current (IPP)||Measured THD (dB)||Clamped
VIN = 40 V)
|SMCJ10CA||10 V||11.1||12.3||17V||2.3nF to 10nF||5µA||88.3A||–69.6||±12.6V|
|SMA6J10A||10 V||11.1||12.3||15.7V||200 to 400pF||5µA||38.2A||–79.5||±12.6V|
|PGSMAJ10CA||10 V||11.1||12.3||17V||80 to 160pF||5µA||23.5A||–81.8||±12.6V|
The following figure shows that the measured THD performance is improved by reducing input signal frequency. This graph was measured with the PGSMAJ10CA TVS diode, RP = 1kΩ, Rflt = 1kΩ, Cflt = 1nF and ADS8588S at 200kSPS on the EVM.
|Device||Key Features||Link||Other Possible Devices|
|ADS8588S||16-bit, 200-kSPS, 8-channel simultaneous-sampling, single-supply bipolar-input SAR ADC||http://www.ti.com/product/ADS8588S||http://www.ti.com/adcs|
|ADS8688||16-bit, 500-kSPS, 8-channel non-simultaneous-sampling, single-supply with bipolar-input SAR ADC||http://www.ti.com/product/ADS8688||http://www.ti.com/adcs|
|ADS8681||16-bit, 1-MSPS, single-channel, single-supply with bipolar-input SAR ADC||http://www.ti.com/product/ADS8681||http://www.ti.com/adcs|
See Analog Engineer's Circuit Cookbooks for TI's comprehensive circuit library.