Current feedback op amps are commonly used as analog to digital converter drivers (A/D or ADC). The following is a chart that lists the key points in designing an A/D driver.
Typically, you will know what frequencies you are interested in feeding into the A/D converter. You will want to have these well within the 0.1dB bandwidth of your op amp. Analog to digital converters are relatively difficult to drive.
- A/D converters or ADCs have 8 to 10 pF of input capacitance and about 600 to 1000 Ohms of input resistance. This makes them prone to peaking and oscillation when driven by an op amp.
- One of the key criteria for an A/D driver is distortion. It is based on the number of bits of the converter. As the information below shows, a 10 bit A/D would require 60 dB of spurious free dynamic range. As you go down to 14 bits, that becomes much more stringent and you need 84 dB of spurious free dynamic range to get the best use of your A/D converter.
- Settling time and settling accuracy are key parameters and for settling time, generally, you want to have it less than half of the time period of your clock. The chart shows how this is figured.
- Settling accuracy for a 10 bit A/D converter needs 0.1%. As you go down to a 14 bit converter, you need 0.006% settling accuracy. This, again, is a very strigent criteria.
- Settling time, as the chart above shows, is 500 nsec for 1 MHz. As you rise to 20 MHz, the converter is down to a 25 nsec settling time.
It's True! The following National Semiconductor part is ideal for these types of applications: LMH6702.
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