High Speed low noise Instrumentation Amplifier with Disable?

Hello I need low noise high speed and high precise Instrumentation Amplifier, my first idea was any as

Figure 68. in datasheet  The 3-op-amp instrumentation amplifiers AD8620+AD8610.

Amplifier is used as differential amplifier with high resistance input. Input signal is sinus, amplitude about 1mV or 2V, frequency up to 1.2MHz Gain=1.

I thing AD8620+AD8610 would be a good choice, unfortunately, needs signals from several amps switch using something like ADG1404.

ADG1404 have crosstalk between chanmel  ~60dB. Consequently, the need to turn off the amplifier output, if not through for ADG1404.

How?

AD8610 have not disable function, my first idea is replace AD8610 with AD8021, ot is good idea?

Any other solving?

 

 

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  • Hi, wivern.

    I have few suggestions that you may consider for your design.

    1. Your 50ohms resistor is too low to be a load of your in amp. This will dissipate a lot of power. It would be better if you increase the resistance and reduce the 1uF capacitor in order to save power and reduce heat and thermal feedback. 

    2. May I know why do you need an in amp as a buffer?

         Since you configured the two sets of your three op amp in amp as buffer, you may try ADI's AD8066 FET input dual amps with a very high impedance and very low input bias current. You may also try AD8170 2:1 buffered multiplexer. The only available JFET op amp with disable that ADI has is ADA4817. It has 1GHz bandwidth which seems an overkill for your application. 

    3. Are you using the fourth channel of ADG1404? If not, is it grounded?

         Based on the posted schematic, you are only using 3channels of the multiplexer. The unused channel must be shorted to ground in order for it to avoid picking up stray signals and to provide DC path for the bias current. Also, floating the unused terminals of a CMOS multiplexer will degrade the performance of the ON-channels.  

    4. ADG1404's off-isolation at 1MHz is more or less 52dB which is high. You may want to replacement it with ADG1204 or ADG5204 which both have higher bandwith and, lower crosstalk and off-isolation (more or less 80dB) at 1MHz.

    5. You may also want to replace ADG1404 with two separate switches or mechanical relays which will not only fully eliminate crosstalk but also will fully turn-off your unused channels without affecting your on-channels.

    Hope this helps. If you still have other questions, just let us know.

    Thanks,

    Anna

Reply
  • Hi, wivern.

    I have few suggestions that you may consider for your design.

    1. Your 50ohms resistor is too low to be a load of your in amp. This will dissipate a lot of power. It would be better if you increase the resistance and reduce the 1uF capacitor in order to save power and reduce heat and thermal feedback. 

    2. May I know why do you need an in amp as a buffer?

         Since you configured the two sets of your three op amp in amp as buffer, you may try ADI's AD8066 FET input dual amps with a very high impedance and very low input bias current. You may also try AD8170 2:1 buffered multiplexer. The only available JFET op amp with disable that ADI has is ADA4817. It has 1GHz bandwidth which seems an overkill for your application. 

    3. Are you using the fourth channel of ADG1404? If not, is it grounded?

         Based on the posted schematic, you are only using 3channels of the multiplexer. The unused channel must be shorted to ground in order for it to avoid picking up stray signals and to provide DC path for the bias current. Also, floating the unused terminals of a CMOS multiplexer will degrade the performance of the ON-channels.  

    4. ADG1404's off-isolation at 1MHz is more or less 52dB which is high. You may want to replacement it with ADG1204 or ADG5204 which both have higher bandwith and, lower crosstalk and off-isolation (more or less 80dB) at 1MHz.

    5. You may also want to replace ADG1404 with two separate switches or mechanical relays which will not only fully eliminate crosstalk but also will fully turn-off your unused channels without affecting your on-channels.

    Hope this helps. If you still have other questions, just let us know.

    Thanks,

    Anna

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