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Voltage shifter circuit

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kathalebm

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Hi All,
I am working on an FPGA that can read either a positive voltage or negative at a time. Since I will use this FPGA to read either +/- voltage, I thought of shifting the -ve voltage to +ve value. I do not know the circuit that I can use to do this. Please, may somebody help me. Thanks in advance.
 

keith1200rs

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What are the two input voltages and what is the I/O voltage of the FPGA?

Keith.
 

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Hi All,
I am working on an FPGA that can read either a positive voltage or negative at a time.
An FPGA can only accept positive values in the inputs so i assume you describe what you want to read and not what an FPGA can actually read .

You have to give more details about the source of these voltages and the nature of them,
are these pulses, what are the expected values etc.

Alex
 

kathalebm

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What are the two input voltages and what is the I/O voltage of the FPGA?

Keith.

Hi Keith,
Thanks so much for your reply. I am using Fusion 484-FPGA to make a simple sampling oscilloscope to measure analog signals that may be of negative or positive voltages. The I/O channels for the FPGA can be configured to measure either +ve or -ve voltage at a time. The other components (Sample and hold) I'm using can work with 5V. Therefore I wanted to clip the signal to be in the range -5V to +5V and shift the -ve voltage to +ve.
If you or any other has an idea on how to go about it is highly welcome to contribute. Thanks in advance.
Benjamin
 

keith1200rs

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I don't know the Fusion FPGA but I would say that you would want to reduce the range as well as shift. So, a -5V input would become 0V, a 0V input would become 2.5V and a 5V input would stay at 5V, for example. Something like this differential amplifier

Keith.
 

kathalebm

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I don't know the Fusion FPGA but I would say that you would want to reduce the range as well as shift. So, a -5V input would become 0V, a 0V input would become 2.5V and a 5V input would stay at 5V, for example. Something like this differential amplifier

Keith.

Hi Keith,
Thanks so much for your contribution. I will look at it. I was looking at Summing Amplifier . The two approaches seem to solve my problem. Thank you.
 

keith1200rs

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Yes, that should do the job. Feed 5V in to the other input and make sure the overall gain is 0.5 should do what you want.

You can actually do it with just a couple of resistors and no opamp but then you can end up with an impedance which is too high for the FPGA ADC. You may be able to get away with it if the signal is only slowly varying and you add a capacitor to ground on the ADC input.

Keith.
 

kathalebm

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Yes, that should do the job. Feed 5V in to the other input and make sure the overall gain is 0.5 should do what you want.

You can actually do it with just a couple of resistors and no opamp but then you can end up with an impedance which is too high for the FPGA ADC. You may be able to get away with it if the signal is only slowly varying and you add a capacitor to ground on the ADC input.

Keith.

Hello sir,
Thank you so much for your contribution. I have tried the two options and all is well. Regards.
 

inventor(y)

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Perhaps this simple one-transistor circuit might work for you:

10_1299203086.jpg



Inventor(y)
 
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