AD8376 cascading and gain seeting

Hi all,

at the moment I am busy designing in AD8376 digital controlled VGA in our application. We want to use this OP in an broad band application which mean a frequency range range between some kHz up to 35 MHz.

When studying datasheet I found some questions where I do not find a satisfying answer for. Hope someone is able to put some light in to the dark.

1. To get a higher dynamic range we want to cascade both OPs. Now I get into trouble concerning the bias point for the 2nd stage. Because we also want so see low frequency signals I have to use some resistors instead of inductors but that would mean the load impdedance for the 1st OP is not 150 Ohm any more. So overall gain setting -4..20dB will be different because of load resistor. Is there any good workaround for that issue?

Using an impdenace converter to decouple is no option.

2. Is it possible to put the output of the first stage without using a capacitor to the input stage of the 2nd one?

3. What about the gain calculation. I found the formula on page 12 of AD8376's datasheet saying gain is load depended. In the overwie on the first page this is decribed as "overall gain setting".

So here are some question to use the formula right:

Only the overall achievable gain is influenced by the load. Increasing load resistor will result in a higher gain but reducing the load will result in a smaller gain. But because of the fact that the passive atteniuator will not changed 1dB steps will still remain the same. Is that correct or will the stepsize due to inner circuits also change when changing load?

Is there any disadvantge if we decide to set a different load to achieve a higher gain over all? May this could be an could option to compensate the drop of first stage.

4. Due to low frequencies we want to use bigger capacitors in series with the input stage. Datasheet only mentions 100nF. Is there any good reason why we should not use ceramic capacitors in range of 1- 10uF?

Thanks in advance for your help.

Any hint is very welcomed

Best Regards

Christian

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  • 0
    •  Analog Employees 
    on Sep 30, 2016 6:25 AM

    Hi, Christian,

    1. The AD8376 was designed to use external pull-up inductors to power the output stage, and the DC level centers around the Vcc level of 5V. 

    2. Replacing the inductors with resistor will lower that a bit, but also eats into the output swing available.  Even so, it's still far from the 1.85V optimum input common-mode level if the 2nd stage is also an AD8376.  A cap would still be needed to isolate the two different common-mode voltages.

    3. The AD8376 has a high output impedance, so we can consider it as a voltage-controlled current source, and the linear gain is proportional to the load resistance.  Halving the load impedance reduces the gain by 6 dB across all gain settings.  E.g., if 75 ohm pull up resistors were used on each output pin, and together with a differential 150 ohm load (2nd AD8376), then gain is reduced by 6 dB -- i.e., -4 to 20 dB range becomes -10 to 14 dB.  Note that maximum output voltage swing will also decrease in this case (probably not quite 6 dB, but we do not have any data on this).

    4. The AD8376 is DC-coupled internally.  There is nothing against using larger coupling caps to reduce the lower corner frequency, just like audio amplifiers.  The reliability of dense (large value in small package) caps vary, so one needs to be careful about sourcing.

    Benjamin

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  • 0
    •  Analog Employees 
    on Sep 30, 2016 6:25 AM

    Hi, Christian,

    1. The AD8376 was designed to use external pull-up inductors to power the output stage, and the DC level centers around the Vcc level of 5V. 

    2. Replacing the inductors with resistor will lower that a bit, but also eats into the output swing available.  Even so, it's still far from the 1.85V optimum input common-mode level if the 2nd stage is also an AD8376.  A cap would still be needed to isolate the two different common-mode voltages.

    3. The AD8376 has a high output impedance, so we can consider it as a voltage-controlled current source, and the linear gain is proportional to the load resistance.  Halving the load impedance reduces the gain by 6 dB across all gain settings.  E.g., if 75 ohm pull up resistors were used on each output pin, and together with a differential 150 ohm load (2nd AD8376), then gain is reduced by 6 dB -- i.e., -4 to 20 dB range becomes -10 to 14 dB.  Note that maximum output voltage swing will also decrease in this case (probably not quite 6 dB, but we do not have any data on this).

    4. The AD8376 is DC-coupled internally.  There is nothing against using larger coupling caps to reduce the lower corner frequency, just like audio amplifiers.  The reliability of dense (large value in small package) caps vary, so one needs to be careful about sourcing.

    Benjamin

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