Using the pre amp of AD8336 as a differential to single ended amplifier

hi Folks

according to AD8336 datasheet, it is a single-ended input and output VGA, but my question is:

1. Is it possible to use its pre amplifier as a differential amplifier to changing a differential input signal to a singled-ended one pass to its VGA part? actually I want to use its pre-amp as a differential to single ended-part of a discrete instrumentation amplifier.

2. the AD8336 datasheet just offered two resistors for 12dB(x4) gain of biasing the pre amp, and said nothing about 26dB(x20) biasing resistor. which resistors are suitable for 26dB gain?

3. are there any app notes about nulling AD8336 offset voltage in high gain? or Do you have any solutions/circuits for doing it?

I highly appreciated to read your comments

TanQ

Parents
  • Hi Memet,

    1) In datasheet says that the preamp is a standard voltage feedback unity gain stable op-amp. Therefore, any standard configurations can be used here.

    actually I want to use its pre-amp as a differential to single ended-part of a discrete instrumentation amplifier

    What do you mean?

    Regards,

    Kirill

  • Dear KirV.

    TanQ for the reply

    in datasheet says these about pre-amp configurations:

    1) page 22. Circuit Configuration for Noninverting Gain: This amplifier is not designed for unity-gain operation. When using ±5 V power supplies, the suggested sum of the output resistor values is 400 Ω total.........  Much of the low gain value device characterization was performed with resistor values of 301 Ω and 100 Ω, resulting in a preamplifier gain of 12 dB (4×).
    With supply voltages between ±5 V and ±12 V, the sum of the output resistance must be increased accordingly; a total resistance of 1 kΩ is recommended.

    2) page 23. Circuit Configuration for Inverting Gain: The considerations regarding total resistance vs. distortion, noise, and power that were noted in the noninverting case also apply in the inverting case, except that the amplifier can be operated at unity inverting gain

    so my question is:

    1. for gain 26dB and supply voltage of ±5V and in noninverting configuration, are 931 and 49 ohm resistor ok or not?

    2. If you look into AD8066 datasheet, you will find below figure(actually fig.60 in page 25.)

    I want to know is it possible to make this circuit by a AD8066 as you can in the figure but replace AD8065 with AD8336's pre-amplifier or not?

    if it is possible what resistors are suitable for biasing the pre-amp of AD8336? 500ohms are ok?


  • I see on your diagram three points of decoupling on AC, therefore all cascades exist separately from each other on DC. Also, you mentioned the required bandwidth from 20 Hertz, so you can use as many capacitors as you want between stages and not worry about nulling.

    I can offer several options. Since the input resistance of the attenuator is known and it is resistive, this resistance can serve as the lower side of the voltage divider. The upper side can be formed by a high-resistance resistor and a potentiometer. 

    You can also add integrators to these stages for automatic nulling and forget about this problem.

  • Dear Kirill

    I hope you be as well as always and thank you so much for your replies.

    ok, So I can add capacitors of any size between the amplifier stages without any worries.. that is a good news :))

    you said: "Since the input resistance of the attenuator is known and it is resistive, this resistance can serve as the lower side of the voltage divider. The upper side can be formed by a high-resistance resistor and a potentiometer. " why did you offer this? you prefer this rather than resistive pi type attenuator?

    at least, about automatic nulling by adding an integrator, would you pls draw a circuit based on AD8336 and an Integrator for automatic offset nulling of it? this is vey helpful for me to complete my idea. TanQ

    Also, If I want to put a side INA topology for input, still AD8066 or single version of it AD8065 is suitable for first stage before AD8336? also do you have any ideas for reducing power frequency interference in the input? still I can keep that difference amplifier topology for  pre-amp of AD8336. does it works?

    
    
  • Hi Memet,

    why did you offer this? you prefer this rather than resistive pi type attenuator?

    No, this does not apply to the input part. This is just one of the possible ways to nulling the offset that I invented and suggested. It does not have any special advantages and looks sophisticated.

    The integrator method can be implemented in this way

    I want to warn you that this method, although very attractive, will give you a headache when implemented. You will need to use high-ohm resistors and large value capacitors to obtain a large time constant that will not affect the bandwidth. Do not rush to apply it. Stick to the decoupling capacitors first.

    how can I coupling the output of AD8336s to AD8302 via ADG1419?
    is the difference topology of the AD8336's pre-amp and its biasing ok or not?

    What's bothering you here? Please explain what you mean by biasing?

    still I can keep that difference amplifier topology for  pre-amp of AD8336. does it works?

    If you can somehow connect the signal source to the amplifier with a symmetrical transmission line, this will be justified. More precisely, the use of INA here will be justified.

    still AD8066 or single version of it AD8065 is suitable for first stage before AD8336?

    I do not know any special advantages of using a dual amplifier instead of two single ones, except for saving space on the PCB and the price.

    is using of AD8066 suitable for this purpose or not?

    Since the input circuits are fairly low-impedance, a bipolar op-amp can give the main advantage here-a lower noise voltage. This is a feature of bipolar op-amps.

    Regards,

    Kirill

  • Dear Kirill

    I highly appreciated for your replies.

    would you pls recommend me some bipolar op-amp as AD8066 substitution with better noise, distortion and voltage offset spec also with sufficient GBW? TanQ

Reply Children
  • Hi Memet,

    Unfortunately, no, it's a bad idea. The data sheet says that it is stable with a gain of -1, which means, in other words, a noise gain of 2. It can't work as a buffer unless you take steps to increase the noise gain. But this doesn't make sense, since it will increase the noise at the amplifier output. Why do you need extra problems?

    I recommend that you study MT-095, which describes how to connect a coaxial cable to an INA.

    https://www.analog.com/media/en/training-seminars/tutorials/MT-095.pdf

    A few more questions: what is the maximum frequency of the calibration signal? And what do the resistors near the analog switch mean?

    Regards,

    Kirill

  • Dear Kirill


    TanQ for the reply.


    first of all, I agree with you about LT1028. I did some simulations. it has terrible response as a unity gain buffer.


    second, how do you calculate the noise gain? and what is the relationship between higher noise gain and stability? is there any document to explain these concepts in detail?


    about your last question:


    1. the maximum frequency of calibration signal is 2 MHz right now.


    2. the resistors near the analog switch, if you meant those series 52.3 Ohm resistors, I had two purpose for putting those. first, as a snob series resistor because of being large ac coupling capacitor after the analog switch and in the input of AD8302. also according to AD8302 datasheet, its input circuit must see the source with 50 Ohm Impedance.
    but I dont really know that it right or not!
    putting that 52.3 Ohm in the input of the analog switch makes a voltage divison by order 2 with the input 52.3 ohm resistor of AD8302. this division is not important for me. it can be compensated by a proper VGA gain.
    but what I want to know is: according to your previous comments, because of being large coupling capacitors in series to the output of the amplifier and not between the output and GND, I can use capacitors as large as I want. but now is it necessary to using 20 to 100 ohm snab series resistor for reducing the peaking of amplitude in the output becoz of being ac coupling large capacitor or not?
    if you look at to my circuit diagram again, you can see 20 to 50 ohm series resistors where I use ac coupling capacitor! can I remove these resistors or not?

    Also, I have a question
    I'm thinking about using a composite amplifier made by a AD8067 and a AD8009 as a substitution of the second AD8336 with a fix gain of 40dB.
    what do you think about the noise, distortion and performance of this combination instead of using AD8336?

    TanQ again for your replies and your patience because of my lots of qestions and also my bad english :)
    Cheers

  • Hi Memet,

    is there any document to explain these concepts in detail?

    Yes, of course

    https://www.analog.com/media/en/training-seminars/tutorials/MT-033.pdf

    the maximum frequency of calibration signal is 2 MHz right now

    You mentioned that the amplitude of this signal is 5 volts, so the RR output is important. I will note that in this case, the slew rate of the amplifier is important, otherwise it will not provide the desired amplitude. Also, the subtractor tries to keep the potential difference between its inputs close to zero, so the first INA stages see a series of 46.4 Ohm resistors as a load. These two input buffers will have to give a significant current to the output, and you will not get a output voltage swing close to the supply rails. Pay attention to the output voltage versus load current graph in the specifications.

    my bad english :)

    I am Russian and live in Russia, so I have no reason to speak English well. My English is much worse anyway. I just use an online translator:)

    Regards,

    Kirill

  • Dear Kirill,

    There is a point.

    We talked so much about possibility of using the INA topology in the input of my desire circuit and went to putting aside INA.

    If I remove the INA topology, Can I connect those passive input circuit s to the +IN pin of AD8336's pre-amp and use it as non-inverting amplifier? Actually using AD8336 pre-amp itselt without any input buffer instead of finding and using a proper single op-amp as a buffer for input...

    What do you think?

    Also I can change my calibration scheme based on this decision if necessary.