The design experience of ADC input protection is too worthwhile after reading it!

When designing ADC circuits, a common question is "how to protect ADC input under overvoltage conditions", then
What problems might arise in an overvoltage situation?

What is the frequency of occurrence?

Are there any potential remedies?

...

In view of the above problems, let us conduct an in-depth analysis!

The overdrive of the ADC input generally occurs when the power rail of the driving amplifier is much larger than the maximum input range of the ADC. For example, the amplifier is powered by ±15 V, while the ADC input is 0 to 5V. The high voltage rail is used to accept ±10 V input while supplying power to the ADC front-end signal conditioning/drive stage. This is very common in industrial design, and this is the case for PLC modules. If a fault condition occurs on the drive amplifier power rail, it can damage the ADC by exceeding the maximum rating, or interfere with synchronization/subsequent conversion in a multi-ADC system.

Although the focus of the discussion here is how to protect precision SAR ADCs, such as the AD798x series, these protection measures are also applicable to other ADC types~

Consider the situation in Figure 1.


Figure 1. Typical circuit diagram of precision ADC design

The circuit above represents the situation in AD798X (such as AD7980) series PulSAR® ADC. There are protective diodes between the input, the reference voltage source and the ground. These diodes can handle large currents up to 130mA, but they only last for a few milliseconds and are not suitable for long periods of time or repeated overvoltages. In some products, such as AD768X/AD769x (such as AD7685, AD7691) series devices, the protection diode is connected to the VDD pin instead of REF. On these devices, the VDD voltage is always greater than or equal to REF. Generally speaking, this configuration is more effective because VDD is a more stable clamp rail and is not sensitive to interference.

In Figure 1, if the amplifier tends to the +15 V rail, the protection diode connected to REF will turn on and the amplifier will try to pull up the REF node. If the REF node is not driven by a strong driver circuit, the voltage of the REF node (and input) will rise above the absolute maximum rated voltage. Once the voltage exceeds the breakdown voltage of the device in the process, the ADC may be damaged. Figure 3 illustrates the case where the ADC driver tends to 8 V and the reference voltage (5 V) is overdriven. Many precision voltage references do not have the ability to sink current, which can cause problems in this situation. Or, the reference drive circuit is very strong enough to keep the reference voltage near the nominal value, but it will still deviate from the precise value.

In a simultaneous sampling multiple ADC system sharing a reference voltage source, the conversion on other ADCs is not accurate because the system relies on a highly accurate reference voltage. If the fault condition takes a long time to recover, the subsequent conversion may also be inaccurate.

There are several different ways to alleviate this problem. The most common is to use Schottky diodes (BAT54 series) to clamp the amplifier output within the ADC range. The related description is shown in Figure 2 and Figure 3. If suitable for application requirements, diodes can also be used to clamp the input to the amplifier.


Figure 2. Typical circuit diagram of precision ADC design
(Added Schottky diode and Zener diode protection)

In this case, the Schottky diode was chosen because it has a low forward voltage drop and can be turned on before the internal protection diode in the ADC. If the internal diode is partially turned on, the series resistance after the Schottky diode also helps to limit the current within the ADC. For additional protection, if the reference voltage source has no/almost no sink current capability, a Zener diode or clamp circuit can be used on the reference node to ensure that the reference voltage is not excessively pulled high. In Figure 2, a 5.6V Zener diode is used for the 5V reference voltage source.


Figure 3. Yellow = ADC input,
Purple = reference voltage source.
The image on the left does not add Schottky diodes,
Schottky diode added to the image on the right


Figure 4. Yellow = ADC input,
Green = ADC driver input,
Purple = reference voltage source (AC coupling)
The image on the left does not add Schottky diodes,
Schottky diode (BAT54S) added to the image on the right

The example in Figure 4 shows the impact on the reference input (5 V) after adding a Schottky diode to the ADC input when the ADC input is overdriven with a sine wave. The Schottky diode is grounded, and the 5 V system rail can sink current. If there is no Schottky diode, when the input exceeds a voltage drop between the reference voltage and the ground voltage, reference voltage source interference will occur. As can be seen from the figure, the Schottky diode completely eliminates the interference of the reference voltage source.

It is necessary to pay attention to the reverse leakage current of the Schottky diode, which can introduce distortion and nonlinearity during normal operation. This reverse leakage current is greatly affected by temperature and is generally specified in the diode data sheet. BAT54 series Schottky diodes are a good choice (maximum 2μA at 25°C, about 100μA at 125°C).

One way to completely eliminate the overvoltage problem is to use a single power rail for the amplifier. This means that as long as the same power supply level (5V in this example) is used for the reference voltage (maximum input voltage), the drive amplifier will never swing below the ground voltage or above the maximum input voltage. If the reference circuit has sufficient output current and drive strength, it can be used directly to power the amplifier. Another possibility is shown in Figure 5, which is to use a slightly lower reference voltage value (for example, 4.096 V when using a 5 V rail) to significantly reduce the voltage overdrive capability.


Figure 5. Typical circuit diagram of single-supply precision ADV design

These methods can solve the problem of input overdrive, but the cost is that the input swing and range of the ADC are limited, because the amplifier has upper and lower margin requirements. Generally, rail-to-rail output amplifiers can be within a dozen mV of power rails, but input margin requirements must also be considered, which may be 1 V or higher, which will further limit the swing to buffer and unity gain configurations. This method provides the simplest solution because no additional protection components are required, but depending on the correct supply voltage, rail-to-rail input/output (RRIO) amplifiers may be required.

The series R in the RC filter between the amplifier and the ADC input can also be used to limit the current at the ADC input during an overvoltage condition. However, when using this method, you need to make a trade-off between current limiting capability and ADC performance. Larger series R provides better input protection, but will cause greater distortion in ADC performance. If the input signal bandwidth is low, or the ADC is not running at full throughput rate, this trade-off is feasible, because the series R is acceptable in this case. The acceptable R size for the application can be determined experimentally.

As mentioned above, there is no way to protect ADC input, but according to application requirements, different individual or combined methods can be used to provide the required level of protection with corresponding performance choices.

Please read the Chinese version for details.