Electronics > Projects, Designs, and Technical Stuff
JFET Buffer Design Issues
madwolfe:
I'm trying to design a high impedance buffer that will go between a DC voltage (between -300mV and 300mV) and an ADC and prevent the ADC affecting the DC voltage (a pH sensor). I would prefer to not have to deal with negative voltages so I'm aiming to offset the pH sensor's voltage.
I'm trying to design a simulation of it using LTSpice and I'm getting an issue where there seems to be some gain and/or low pass filtering on the output. I've tried changing around various things but I'm still getting the issue. When I supply the buffer with +-5V and remove the offset to the voltage source, it buffers fine. I've attached an image of my setup, any insight would be very much appreciated, I feel like it's something quite basic that I'm missing.
madwolfe:
That's silly of me - I just double checked what the input voltage range was and it looks like the ADA4610 is dual supply only :palm:.
However, if I configure the offset to be +5V and the supply to be 10V, it works fine. Lesson learnt: read the datasheet properly.
TheHolyHorse:
--- Quote from: madwolfe on November 13, 2019, 09:42:38 am ---That's silly of me - I just double checked what the input voltage range was and it looks like the ADA4610 is dual supply only :palm:.
However, if I configure the offset to be +5V and the supply to be 10V, it works fine. Lesson learnt: read the datasheet properly.
--- End quote ---
You can run it from a single supply just fine just make sure you're within the common mode voltage range.
Two 12V sources in series is exactly the same as one 24V source. So dual supply only isn't really a thing.
You might still want 2 supplies tho, depending on what you're doing.
madwolfe:
--- Quote from: TheHolyHorse on November 14, 2019, 07:01:27 am ---
--- Quote from: madwolfe on November 13, 2019, 09:42:38 am ---That's silly of me - I just double checked what the input voltage range was and it looks like the ADA4610 is dual supply only :palm:.
However, if I configure the offset to be +5V and the supply to be 10V, it works fine. Lesson learnt: read the datasheet properly.
--- End quote ---
You can run it from a single supply just fine just make sure you're within the common mode voltage range.
Two 12V sources in series is exactly the same as one 24V source. So dual supply only isn't really a thing.
You might still want 2 supplies tho, depending on what you're doing.
--- End quote ---
Very true, but I'm stuck with using a 5V supply so I would need a +-2.5V opamp which is a bit more limiting. I've found the TL092 or TL2072, both of which seem to fit my buffer requirements ok and are good for a 5v single sided supply. I'm then using a MCP1501 to offset the input voltage to make it positive.
That being said, my aim is to provide a high input impedance reading with an ADC, how does having a series voltage reference affect the impedance as seen by the voltage-generating sensor?
SiliconWizard:
You could consider something like the OPA140? http://www.ti.com/product/OPA140
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