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High voltage variable gain instrumentation amplifier.

Category: Hardware
Product Number: LTC6090 and ADA4254

Hi folks,

I am currently using the LTC6090 in a precision, unity gain instrumentation amplifier mode at its maximum operating voltage (with a voltage limiter) to drive a ADA4254 VGA (in a prototype system).  This is a DC coupled system, with relatively low bandwidth (<10kHz).  Unfortunately, the ADA4254 does not have the same voltage range as the LTC6090, and as a result I am not able to use the full dynamic range of my system.  In my application, the dynamic range of the system is quite large, and as a result there are situations where I will need to precisely amplify or attenuate the signal coming out of the LTC6090 before going into an ADC. The architecture of the ADA4254 provides the gain range choices I need, but not the voltage range; it is a +/-28V circuit. 

I have looked at a variety of different topologies for digitally controlled variable gain instrumentation amplifiers, but all of them are oriented around low voltage gain control circuits, or have the potential for high gain error.  Thus far, I have not been able to find another amplifier circuit/product that does both gain and attenuation in the voltage range I am using.  I would like to find another digitally controlled, instrumentation amplifier circuit/product that allows me to use a the full dynamic range of the LTC6090. 

Or, is this something I am going to have to design from scratch?

Eventually, I would like to transition to using the ADHV4702-1, which has an even higher voltage requirement.

Do you have any suggestions?



  • Hi Allan,

    I have looked at a variety of different topologies for digitally controlled variable gain instrumentation amplifiers, but all of them are oriented around low voltage gain control circuits, or have the potential for high gain error

    For the Instrumentation Amplifier portfolio, ADA4254 has the highest output capability about +/-25V(with headroom). Although I'm not quite familiar with the VGA portfolio (I only handle InAmp), looking at the parametric search, VGA's highest output capability is along 18-20V (AD8336) only. Does your voltage requirement need to be 140V similar to LT6090? or just anything >56V up to 140V?

    but all of them are oriented around low voltage gain control circuits

    Would you be able to share up to what gain level do you wish your system would be?

    For now, with the high voltage range of your requirement, I'm thinking the solution we would be offering is discrete. I'll look into this more.



  • Hi Kristine,

    Thank you for your quick response.  We are using the ADA4254 now; it's the only commercially available product we have been able to find, thus far, that is remotely capable of being useful in our product.  Its gain range is highly valuable for what we are doing.  Unfortunately, the ADA4254 does not have a high enough input voltage range for us to fully exploit the capability of our system.  It is working okay for now, but we would like to be able to use the full capability of our design.  Because of the very high dynamic range in the system we need to be able to amplify and attenuate the signal like the ADA4254 does. 

    The ADA4254 is a really awesome product; however, we need to have something like the ADA4254, but with an input voltage range of about +/-70V or about 3x the ADA4254 capability.  The similar product, ADA4255, goes the wrong way from an input voltage range perspective.

    Because of the ADA4254 architecture, I don't think it's possible to use bootstrapping to boost the supply voltages, but you would be in a better position than me to define whether that is possible.

    Got any other ideas?

    Thank you,


  • Hi Kristine,

    Sorry for the late reply; I've been a bit busy lately, and I forgot to answer part of your question until I returned to the original problem from last year.  In actuality, we would like to have a precision variable gain instrumentation amplifier that works up to the full voltage range of the ADHV4702-1 or higher, which is +/-110V.  Interestingly enough our dynamic range is very close to the +/-110V supply maximum for the ADHV4702-1 all the way down to <+/-0.01V.  The high voltage is required for a variety of reasons.  The ADA4254 is still the only product out there that gets anywhere near the performance we need without extremely high costs. 

    I like the idea behind the ADA4255, but it looks like the charge pump actually reduces the dynamic range of the product relative to the ADA4254.  Unfortunately, there is no charge pump override for the VDDH and VSSH supplies so that they can be externally applied, if the additional +/-5V or so of headroom are needed. 

    The problem for me is that the meaning of "high voltage" now days is pretty much anything above 12V.  So, I wind up screening through a bunch of products that are unsuitable for the task at hand.  Additionally, pretty much most all low voltage variable gain amplifier solutions in either the combination discrete ideas arena or integrated product offerings will not be able to work at the high voltages we need, or provide the gain range.  We have to be able to attenuate the high voltages or amplify the small signals as needed, depending on the signal amplitude at the VGA inputs. This instrumentation amplifier problem stems from the high dynamic range of this system, which is >10,000, including high the voltages. 

    Got any ideas?