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ADS8329: When are Op-Amp drivers necessary for the analog inputs?

Part Number: ADS8329
Other Parts Discussed in Thread: LMV1032, OPA350

The datasheet for the ADS8329 states that the analog inputs need to be driven by op-amps. It doesn't give any sort of conditions or reasoning for the op-amps and all I have are assumptions.

I was wondering if the inputs could be driven by the LMV1032 mic pre-amplifier since my application requires the part anyway. This would save money, board space, and power; the most important of which are power and space.

If I can drive the inputs of the ADS8329 with the LMV1032, is there anything that should be taken into consideration when connecting the two? (i.e. DC bias)

Thanks in advance,

Chris

  • When connected to an ADC, any signal almost always needs to be scaled and shifted to the correct range for the ADC.

    In this case, the LMV1032 has an output that is biased at the center of the signal amplitude, and the ADS8329 can use a reference that would take care of the scaling for you, while also making the voltage shifting unnecessary. That is, if you can easily supply the correct voltages for +Ref, -Ref, and +VA. You would need to look at your design and determine if it may be easier to use an intermediate amplifier.

    Another consideration has to do with the inner workings of a Successive Approximation Register (SAR) ADC. The input is basically a switched capacitor, switching at the rate of CONVST. From the datasheet, this capacitor is about 40pF, and at 1MSPS that works out to 1 / (40pF * 1MSPS) = 25kOhms. It might seem like an LMV1032 should be able to drive a 25kOhm load, but the difficulty is that the load is capacitive. To help with this, you will need resistor in series with the LMV1032's output, and a capacitor to ground. The capacitor will help to quickly supply charge to the switched load, while the series resistor will be needed because the LMV1032 will not be able to drive it directly. Even with the correctly designed RC circuit at the input of the ADC, and especially at higher sample rates like your ADC is capable of, we often find that a higher bandwidth driver amplifier is necessary. Slowing down the converter's sample rate eases the demands on the driving circuitry, so there is usually some sample rate that will accurately measure your input signal. (This is where the Nyquist sampling theory may come into play.)

    In your application, without an intermediate ADC driver amplifier, you most certainly will not be able to achieve the best performance at the maximum sample rate. How much sample rate reduction you will need in order to achieve acceptable performance would be difficult to determine.

    I would recommend that you consider the OPA350 as a possible intermediate driver for your ADC. It has the bandwidth to do the job.

    In other words, to answer your question, an op-amp driver is necessary when your application makes it necessary. They help to do the difficult task of driving the SAR input circuitry, enabling performance at fast sample rates, while doing all of the shifting and scaling that many applications need. On the other hand, as you point out, not including one could potentially save money, board space, and power. It is up to the designer to balance these trade-offs.
  • Thanks for the quick response. This answer helps a lot.

    Luckily the sample speed I need is actually about 1/10 the max sample rate. I'll give it a go with and without the driver and I'll post the results when I get them.

    Thanks again,

    Chris

  • With a lower sample rate, there is hope that you might not need a driver amplifier.  With a SAR ADC from TI, any time the converter is not actually converting, it is in Acquisition Mode -- meaning that the sampling capacitor is connected to the input, and your input drive circuit is charging it.  (There is some series switch on-resistance involved as well, giving it an RC time constant.)  As long as you have enough acquisition time, your drive circuit will charge the sampling cap to less than 1/2 LSB from the input voltage, and you will get an accurate conversion.  Less acquisition time (faster sampling rate) means you need a faster drive circuit to accomplish this. 

    So when you need an op-amp driver is a question with a pretty complicated answer.