Re: Strange ADC buffer amp design

From: Ban (bansuri_at_web.de)
Date: 08/01/04 

Date: Sun, 01 Aug 2004 13:08:19 GMT

Stefan Heinzmann wrote:
>>>
>>> Appendix A Figure 24 shows the recommended input buffer schematics
>>> which seems rather odd to me. Ok, it sorta works up to 100kHz, but
>>> it looks rather unbalanced to me, with hints of phase problems. Is
>>> there anything to be said in favour of it that I may have
>>> overlooked? My own solution would look a lot more symmetrical, where the
>>> inverting amplifier picks up its input from the signal source
>>> rather than the output of the noninverting amp. I lashed up a
>>> simulation of both circuits and on first glance mine looks a lot
>>> cleaner. Anything specific I need to watch out for?
>>>
>>> I had no luck so far getting useful information from Cirrus btw.
>>
>>
>> Stefan,
>> I like those filters. the follower has 10k input impedance, whereas
>> the inverter is very low (634R) to maintain the low noise of the
>> DAC. The output phase compensation is needed because of the large
>> 2700p filter cap. So the whole filter is using the internal
>> resistance plus 91R vs. 2n7*2 to gnd to form a first order LP, which
>> rolls off at 300k. At higher frequencies the internal resistance
>> goes up, so the filter is actually of 2nd order, but at lower
>> frequencies it behaves only like a first order filter.
>
> I thought that because of the resistive feedback being picked off
> behind the 91 Ohm resistor, the actual output impedance feeding the
> filter cap is a lot lower than 91 Ohm, so the LP corner frequency
> would be much higher. The 91 Ohm resistor together with the feedback
> capacitor clearly is meant to prevent instability due to the large
> capacitive load. I don't think it has much to do with LP filtering.
>
Yeah, the transition is soft, because with this compensation and the huge
capacitive load the O/P resistance will rise depending on the opamp used to
pretty high values from 50kHz on, and at 300k it will be probably even more
than 100R, because the 470p will reduce the gain at high frequencies. It
then stays constant until the ft-limitation kicks in. The 2700p across the
A/D inputs will act as an integrator together with this resistance. It is
needed because the chip uses switched sampling capacitors and those are
charged from this big cap.

>> The 2n7 cap also gives a symmetric filter response without using two
>> different caps, since both ends are symmetrical. At higher
>> frequencies there will be some slight assymetric behaviour, since
>> one output is loaded with 634R, whereas the other with 634+91R. But
>> what is important is the absolutely symmetric phase shift. Quite
>> smart.
>
> I would have thought that picking the inverter's input from ahead of
> the 91 Ohm resistor would cause phase and level problems at higher
> frequencies. But thinking of it they will be high enough in frequency
> as to be irrelevant.
>

It is not a good circuit, you are right, seems to be made by a digital
designer.

>> I would recommend a quad AD8674 for the opamp, So both channels share
>> similar characteristics.
>> Could you post your design as well to make a comparison.
>
> Here is the LTspice schematics. I chose the LMV721 because it can
> drive the capacitive load without resorting to isolation resistors,
> which saves me a few parts. You would have to get the model from
> National if you wanted to simulate it. The input impedance is also
> 10k, as you can observe. R9 is the impedance internal to the CS42428,
> according to the data***. V2 represents the VQ output of the
> CS42428, which I chose as the biasing source, but it needs buffering.
> I haven't yet looked at the noise, which may be the major catch.
>

Well, if you want to save parts, you better not do it that way :-)
The noise will be around 4 to 5 times(!) higher than what I drew here:

    ||100u |\ 3/4 AD8674
 o--||--+------|+\ |\
    || | | >--+------|+\ ___
       .-. +-|-/ | | >--+-|___|-+------+-o
    10k| | | |/ | +--|-/ | 91R | |
       | | | | | |/|| | | |
       '-' +-------+ +----||--+ | |
        | | | || | |
        | | | ___ | |
        | | +--|___|---------+ |
        | | 475R ---
        | | ___ 2700p ---
        | .-. +--|___|---------+ |
        | | | | 475R | |
        | 475R| | | || | |
        | '-' +----||--+ | |
        | | | |\|| | | |
        | +---+--|-\ | ___ | |
        | | >--+-|___|-+------+-o
        +---------+---------|+/ 91R
        | | + |/
        | ###
        oUref ---
                  |
                 ===
                 GND
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