When designing an analog-to-digital converter (ADC) circuit, one of the most common challenges is ensuring proper protection for the ADC input under overvoltage conditions. This issue can arise in a variety of scenarios and requires careful consideration of the system's power supply, signal conditioning stages, and reference voltages. While different ADC manufacturers may implement slight variations in their protection schemes, the underlying principles remain largely consistent. This article explores the potential risks of overvoltage events, how frequently they occur, and effective solutions to safeguard precision ADCs such as the AD798x family, while also addressing other ADC types.
Overvoltage on an ADC input typically occurs when the driver amplifier operates at a higher voltage than the ADC’s input range. For example, if the amplifier is powered by ±15 V but the ADC accepts a 0–5 V input, a fault condition could cause the amplifier output to exceed the ADC’s maximum allowable voltage. In industrial applications, such as programmable logic controllers (PLCs), it's common to use high-voltage rails to accommodate larger input signals, which can create a risk of damaging the ADC if not properly managed. If the driver amplifier fails or experiences a fault, the ADC could be damaged due to excessive voltage or disrupted timing in multi-ADC systems.
Consider the typical circuit shown in Figure 1. This diagram represents a precision ADC design, such as the AD7980 series. A protection diode is usually placed between the ADC input, the reference, and ground. These diodes are designed to handle currents up to 130 mA but only for short durations—making them unsuitable for repeated or prolonged overvoltage events. Some devices, like the AD768X/AD769X series, connect the protection diode to the VDD pin instead of the reference. This configuration is generally more robust because VDD is a stable rail that is less prone to interference compared to the reference node.
In the circuit from Figure 1, if the amplifier is connected to a +15 V rail, the protection diode tied to the reference could activate, causing the reference node to rise above its rated voltage. If the reference is not driven by a strong buffer, this could lead to damage to the ADC. Many precision references lack the ability to sink current, which can exacerbate the problem. Even with a strong drive, the reference voltage might still deviate slightly from its nominal value, affecting the accuracy of the ADC.
In multi-ADC systems where a single reference is shared, overvoltage on one channel can cause inaccuracies in other ADCs. If the recovery time after a fault is long, subsequent conversions may also be affected, leading to unreliable results.
To mitigate these issues, several strategies can be employed. One common approach is to use a Schottky diode, such as the BAT54 series, to clamp the amplifier output within the ADC’s input range. This helps prevent the internal protection diodes from being overstressed. Additionally, a Zener diode or clamp circuit can be added to the reference node if it lacks sufficient sinking capability. Figure 2 shows an example of a circuit incorporating both Schottky and Zener diode protection. The Schottky diode is chosen for its low forward voltage, allowing it to activate before the internal ADC diodes. It also helps limit current flow, reducing the risk of damage during overvoltage events.
Figures 3 and 4 illustrate the impact of adding a Schottky diode to the ADC input. Without the diode, the reference voltage can be pulled beyond its safe operating range, potentially damaging the ADC. With the diode in place, the reference remains stable, ensuring accurate and reliable operation even under fault conditions.
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