I have got quite a few questions over the years about how to have an effective mechanism to protect the inputs of the Analog-Digital Converter (ADC). We have done some experiments in the past where we tried various things. All of them almost invariably ended up disrupting the ADC's normal performance. The protection mechanism almost always worked (as in it squelched the surge in input signal), but it made the ADC performance look horribly bad in normal operation.
The holy grail here was to have a
- fast acting diode that will clamp the ADC input voltages so it does not over stress the ADC input circuitry
- be benign during normal operation so that the ADC performance is not affected
As we have gone to faster and faster sampling rates (with RF-Sampling ADCs) the need for having these protection components is even more critical. In some applications the ADC inputs use a passive front end that is beneficial for a low noise performance of the ADC. However, this leaves the ADC inputs sort of vulnerable to sudden surges in input voltage. This does not necessarily damage the ADC inputs, but depending on the ADC design, could affect the reliability (longevity) of the ADC.
I have been able to find passive devices that fit the "holy grail" conditions listed above. The secret lies in careful selection and evaluation of the diodes. Fast acting Schottky diodes are beneficial because of the low junction capacitance. This low capacitance helps in maintaining the normal ADC performance. The clamping voltage is important since this determines at what voltage the diode will clip the signals going to the ADC.
There is a new family of Amplifiers from ADC that have a "Fast Attack" feature. This feature is primarily intended for AGC (Automatic Gain Control). When asserted, the gain of the amplifier get clipped to a preset level. However, there is a latency associated with the Fast Detect output from the ADC triggering the Fast Attack bit of the amplifier (shown in figure 1).
Figure 1 : ADA4961 driving the AD9680 showing RF-sampling ADC and Schottky diodes.
Careful selection of the Schottky diode can protect the ADC inputs, allowing a system designer to implement an amplifier driven front-end circuit using the latest fast-attack and fast-detect features.
The full transcript of the paper can be found here : RF-Sampling ADC Input Protection : Not Black Magic After All
I would be interested to have your feedback/comments. Is this something that has been one of your concerns in system level design?