Take out your notebook and write down these ADC input protection design experiences

Overdriving the ADC inputs typically occurs when the driver amplifier rail is much larger than the ADC maximum input range, for example, the amplifier is powered at ±15 V and the ADC input is 0 to 5 V. It is common in industrial designs for high voltage rails to accept ±10 V inputs while also powering the ADC front-end signal conditioning/driver stages, as is the case with PLC modules. If a fault condition occurs on the driver amplifier rail, it can damage the ADC by exceeding the maximum ratings or interfere with simultaneous/subsequent conversions in a multi-ADC system.

Although the focus here is on 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 for precision ADC design.

The circuit above represents the situation in the AD798X (e.g. AD7980) family of PulSAR® ADCs. Protection diodes are present between the inputs, the reference, and ground. These diodes are capable of handling large currents up to 130mA, but only for a few milliseconds and are not suitable for longer or repeated overvoltages. On some products, such as the AD768X/AD769x (e.g. AD7685, AD7691) family of devices, the protection diodes are 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 efficient because VDD is a more stable clamping rail that is less sensitive to interference.

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

In a simultaneous sampling multi-ADC system that shares a common voltage reference, conversions on other ADCs will be inaccurate because the system relies on a highly accurate reference voltage. Subsequent conversions may also be inaccurate if the fault condition recovery time is long.

There are several different ways to mitigate this problem. The most common is to use a Schottky diode (BAT54 series) to clamp the amplifier output to the ADC range. This is illustrated in Figures 2 and 3. Diodes can also be used to clamp the input to the amplifier if this is appropriate for the application.

Figure 2. Typical circuit diagram for precision ADC design.

(Schottky and Zener diode protection added)

In this case, the Schottky diode was chosen because it has a low forward voltage drop and can turn on before the internal protection diodes in the ADC. The series resistor after the Schottky diode also helps limit the current in the ADC if the internal diode turns on partially. For additional protection, if the reference has little or no current sinking capability, a Zener diode or clamp circuit can be used on the reference node to ensure that the reference voltage is not pulled too high. In Figure 2, a 5.6V Zener diode is used for a 5V reference.

Figure 3. Yellow = ADC input,

Purple = voltage reference.

The left image does not have the Schottky diode added.

The right image has a Schottky diode added

Figure 4. Yellow = ADC input,

Green = ADC driver input,

Purple = Reference (AC coupled)

The left image does not have the Schottky diode added.

The right image has a Schottky diode (BAT54S) added

The example in Figure 4 shows the effect of adding Schottky diodes to the ADC inputs on the reference input (5 V) when overdriving the ADC inputs with a sine wave. The Schottky diodes are connected to ground and the 5 V system rail is able to sink current. Without the Schottky diodes, reference glitches would occur when the input exceeds the reference voltage and ground by a voltage drop. As can be seen in the figure, the Schottky diodes completely eliminate the reference glitches.

It is important to be aware of the reverse leakage current of the Schottky diode, which can introduce distortion and nonlinearity during normal operation. This reverse leakage current is highly temperature dependent and is generally specified in the diode data sheet. The BAT54 series Schottky diode is a good choice (2μA maximum at 25°C, approximately 100μA at 125°C).

One way to completely eliminate overvoltage issues is to use a single supply rail for the amplifier. This means that as long as the same supply level (5 V in this case) is used for the reference voltage (maximum input voltage), the driving amplifier will never swing below ground 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), thereby significantly reducing the voltage overdrive capability.

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

These methods solve the input overdrive problem, but at the expense of limited input swing and range of the ADC due to headroom and headroom requirements on the amplifier. Typically, rail-to-rail output amplifiers can be within a few 10 mV of the rails, but input headroom requirements must also be considered, which can be 1 V or more, further limiting the swing to a buffer and unity-gain configuration. This method offers the simplest solution as no additional protection components are required, but it relies on the correct supply voltage and may require a rail-to-rail input/output (RRIO) amplifier.

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, there is a trade-off between current limiting capability and ADC performance when using this method. A larger series R provides better input protection but results in greater distortion in the ADC performance. This trade-off is acceptable if the input signal bandwidth is low or the ADC is not running at full throughput because the series R is acceptable in this case. The size of R that is acceptable for the application can be determined experimentally.

As mentioned above, there is no one-size-fits-all approach to protecting ADC inputs, but depending on the application requirements, different approaches, either alone or in combination, can be used to provide the required level of protection with corresponding performance trade-offs.