Voltage to frequency converter (VFC) circuit

[justanengineer]

Hello,

i want to implement a Voltage to Frequency Converter that convert Voltages in order of 10µV in a frequency band ( 0.1-10kHz), with very low noise (order of some µVrms ).

Is that possible?

Thanks

 

[barry]

You want a 10uV range and "some uVrms" of noise? Like 9uV of noise on a 10uV range? Or something different? What is your actual range?

Without knowing more, I'd say you've got a near-impossible requirement there.

What is your voltage range? What is your accuracy requirement? If it's, say, 100uV full-scale, that means you've got a sensitivity of 100KHz/100uV==> 100MHz/Volt. That's pretty sensitive. Said another way, 1uV of noise will cause 100Hz deviation.

 

[justanengineer]

i have to use my VFC for Neuronal recording system, so the signal recieved from the electrode is from uV till some mV, so the noise should be less than 5 uV.
Then i was thinking to put a common source stage before the VFC to amplify a little the small input signal and like that i have more margin.
It could work?

 

[barry]

You can definitely add an amplifier, but you'll also be amplifying the noise. You need to be a little more specific than "some mV". That is not a good way to go about designing something. Is it 2 mV? 900mV? That's a big difference. You need SOME specifications. What kind of frequency response is acceptable? Can you add some filtering? Averaging?

 

[justanengineer]

The range in amplitude of the extracellular biopotentials are typically from 10μV to about 5mV, with power in the 10-200Hz band for LFPs(Local field potentials) and 200Hz-5kHz band for action potentials.
So to record the signal from the brain, any topology in use should have an input referred nois less than 5uV.
It's possible to implement a VFC that can convert small Voltage signal (10uV) from an electrode to Frequency??

 

[Audioguru]

You need to amplify the brain waves because you sense them far from the brain with lots of skin in between attenuating the signals. If you probe directly inside the brain then the signal levels will be much higher and might be able to drive your VFC without an amplifier.

An ECG waveform has the same problem and also needs an amplifier.

 

[barry]

I don't have any experience with bio-engineering, but my first thought is that I would amplify the signal(using a low-noise amplifier) from the sensor as close to the sensor as possible. That way, the amplified signal will be less susceptible to external noise as it's carried over the cable from the sensor to your circuit.

But I don't know about designing a VFC that will work directly from such low voltage inputs.

 

[justanengineer]

I will use my circuit for IN VITRO measurement. And i was thinking that instead of using the MEA (MicroElectrode Array ) topology, to use a VFC like that the elaboration outside the array will be more easy, but i am still not sure if we can implement this VFC in such a way to convert input signal in order of some uV.