Hi Guys,
This is quite an open ended question, and I'm being vague on purpose. Please no replies with "give more details of circuits" - there is no circuit at the moment.
I have a project on which involves measuring the impedance of a material using a sinusoidal signal at 18MHz. I plan to feed a stable excitation signal into the material (known f & V), and then determine Z by measuring the current flow. I need to achieve great stability and accuracy over a wide temperature range (-20 degrees to +120 degrees). All of the electronics in the circuit are going to be exposed to this range of temperature. I don't want the analogue measurement to be affected by resistors changing value etc as ambient temperatures change.
At the moment, current techniques involve profiling the measurement boards over temperature, then normalising the effect out via firmware look-up tables. What I'd like to achieve is a reading stable enough over temperature that this step is not required. And so, my question:
To any experienced analogue design engineers out there, what techniques do you employ to ensure accurate measurements over temperature? My current thinking is as follows:
> Achieve a rock-solid excitation frequency by using a low ppm xtal fed through a DDS
> Use something like a 2.5V voltage-reference IC that's stable over temperature to ensure a stable excitation voltage
> Currently unsure how to measure current with such a high excitation frequency, with stability over temperature. A very fast transimpedance amplifier with fast rectification and filtering?
Any hints/tips/help you guys can throw my way is greatly appreciated.
Dan
This is quite an open ended question, and I'm being vague on purpose. Please no replies with "give more details of circuits" - there is no circuit at the moment.
I have a project on which involves measuring the impedance of a material using a sinusoidal signal at 18MHz. I plan to feed a stable excitation signal into the material (known f & V), and then determine Z by measuring the current flow. I need to achieve great stability and accuracy over a wide temperature range (-20 degrees to +120 degrees). All of the electronics in the circuit are going to be exposed to this range of temperature. I don't want the analogue measurement to be affected by resistors changing value etc as ambient temperatures change.
At the moment, current techniques involve profiling the measurement boards over temperature, then normalising the effect out via firmware look-up tables. What I'd like to achieve is a reading stable enough over temperature that this step is not required. And so, my question:
To any experienced analogue design engineers out there, what techniques do you employ to ensure accurate measurements over temperature? My current thinking is as follows:
> Achieve a rock-solid excitation frequency by using a low ppm xtal fed through a DDS
> Use something like a 2.5V voltage-reference IC that's stable over temperature to ensure a stable excitation voltage
> Currently unsure how to measure current with such a high excitation frequency, with stability over temperature. A very fast transimpedance amplifier with fast rectification and filtering?
Any hints/tips/help you guys can throw my way is greatly appreciated.
Dan
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