【Practical】Design experience of ADC input protection

Overdrive on the ADC input typically occurs when the driving amplifier rails are much larger than the ADC's maximum input range. For example, the amplifier is powered by ±15 V and the ADC input is 0 to 5V. The high voltage rails are used to accept the ±10 V input while powering the ADC front-end signal conditioning/driver stage, which is common in industrial designs, as is the case with PLC modules. If a fault condition occurs on the driver amplifier rails, it can damage the ADC by exceeding maximum ratings, or interfere with synchronization/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

上图电路代表AD798X（例如AD7980）系列PulSAR® ADC中的情形。输入端、基准电压源和接地之间存在保护二极管。这些二极管能够处理最高130mA的大电流，但仅能持续数毫秒，不适用于较长时间或重复过压。在一些产品上，例如AD768X/AD769x（如 AD7685、AD7691）系列器件，保护二极管连接至VDD引脚而不是 REF。在这些器件上，VDD电压始终大于或等于REF。一般而言，此配置更有效，因为VDD是更稳定的箝位电轨，对干扰不敏感。

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 system of simultaneously sampling multiple ADCs that share a common reference, conversions on the other ADCs are inaccurate because the system relies on a highly accurate reference. If the failure condition takes longer to recover from, subsequent conversions may also be inaccurate.

缓解此问题有几种不同方法。 最常见的是使用 肖特基二极管 （BAT54系列），将放大器输出钳位在ADC范围。相关说明详见图2和图3。如果适合应用需求，也可使用二极管将输入箝位在放大器。

Figure 2. Typical circuit diagram for precision ADC design.

(Schottky and Zener diode protection added)

在此情况中， 之所以选择肖特基二极管，是因为其具有低正向导通压降，可在ADC内的内部保护二极管之前开启。 如果内部二极管部分开启，肖特基二极管后的串联电阻也有助于将电流限制在ADC内。对于额外保护，如果基准电压源没有/几乎没有灌电流能力，则可在基准节点上采用齐纳二极管或箝位电路，以保证基准电压不被过度拉高。在图2中，为5V基准电压源使 用了5.6V齐纳二极管。

Figure 3. Yellow = ADC input,

Purple = reference voltage source.

The image on the left has no Schottky diode added,

Image on right with Schottky diode added

Figure 4. Yellow = ADC input,

Green = ADC driver input,

Purple = Reference (AC coupled)

The image on the left has no Schottky diode added,

The image on the right 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).

完全消除过压问题的一种方式是为放大器使用单电源电轨。 这意味着，只要为基准电压（最大输入电压）使用相同电源电平（本例中为5V），驱动放大器就绝不会摆动至地电压以下或最大输入电压以上。如果基准电路具有足够的输出电流和驱动强度，则可直接用来为放大器供电。 图5中显示了另一种可能性，也就是使用略低的基准电压值（例如，使用5 V电轨时为 4.096 V），从而显著降低电压过驱能力。

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

These methods can solve the problem of input overdrive, but at the cost of limiting the input swing and range of the ADC due to the headroom and headroom requirements of the amplifier. Typically, rail-to-rail output amplifiers can be within a dozen mV of the rails, but input headroom requirements must also be considered, which may be 1 V or more, which further limits the swing to buffer and unity-gain configurations. This approach provides the simplest solution as no additional protection components are required, but relies on the correct supply voltage and possibly 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.