ADC128S102: Protection diode

daejin kang

Part Number: ADC128S102

hi, Im daejin

I know the absolute maximum ratings of analog inputs. (−0.3 VA +0.3)

Therefore, I configured a clipping circuit to protect the ADC from the failure mode of the analog input (as in Figure 38. Typical Application Circuit).

However, the schottky diode that I used distorts the analog input signal due to leakage current. (At high temperatures, 60 degrees Celsius)

Is there a way to solve this?

And for the schottky diode (SBR0220T5) shown in "Figure 5-1 ADC128S102EVM Schematic" (in ADC128S102EVM Booster Pack User's Guide), the forward voltage is more than 0.3 V.

If the schottky diode is used, it will exceed the ADC's maximum rating (−0.3 VA +0.3).

Nevertheless, is there a reason for using the schottky diode(SBR0220T5)?

Thank you.

over 4 years ago

0 Cynthia over 4 years ago

TI__Mastermind 24940 points

Hello,

The Schottky diode is added to protect the input from possibly going past the VA.

This is a precaution some customers feel is necessary in their circuits. This is usually when there are voltages present in the circuit that are higher than VA.

If the highest voltage level in your system is 5V, then it might not be necessary to use a protection diode.

We have a great video series on the input protection of ADCs, which also speaks to the use of Schottky diodes, TIPL: Electrical Overstress on Data Converters

This series speaks to exactly what you are seeing and address how to mitigate it.

Regards

Cynthia

0 daejin kang over 4 years ago in reply to Cynthia

Intellectual 320 points

Thank you for answer.

The maximum voltage of my system is -12 ~ + 12 V (failure mode).

The VA of ADC is 5 V, my system's nominal analog input voltage range is 0 ~ + 5V.

But I'm worried about failure mode, so I will use a schottky diode.

Therefore, I am considering about signal distortion due to leakage current. (1n5822)

also, In case of using a schottky diode (SBR0220T5), when in failure mode, analog input voltage will exceed the maximum rating of the ADC (-0.3VA +0.3).

thank you in advanced.

daejin

0 Cynthia over 4 years ago in reply to daejin kang

TI__Mastermind 24940 points

Hello,

There has been new methods and advanced in diodes since the device has been release.

The first being that there are and improved diodes that have much lower leakage current. I would suggest looking at BAT54 or BAT42 which have very low leakage current compared to the diode listed in the datasheet.

If this still does not meet your system requirements, there are some circuit changes implementations that further improve the diode interference, if you are open to that.

The first method is to add a second resistor after the diode along with the capacitor. this is explained in TI Precision Labs –Protecting Low Voltage ADC from High Voltage Amp

A more complex method is to move the first resistor within the amplifier feedback loop. this is explained in as well as the following cookbook

Circuit for protecting low-voltage SAR ADC from electrical overstress with minimal impact on performance or in 8.5 TI Precision Labs – Protecting Low Voltage ADC - Improved Solution

Regards

Cynthia

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ADC128S102: Protection diode