# Step Down Voltage Circuit at High Frequency (MHz)

1. ## Step Down Voltage Circuit at High Frequency (MHz)

Hello everyone,

I have a 13.56 MHz signal value between 80-100 V. I need to translate these voltages to 0-5 V for my microcontroller with the highest resolution. I understand that I can step down the voltage using a transformer. However, I would like to know an analog circuit example in which I can use instead a transformer.  Reply With Quote

2. ## Re: Step Down Voltage Circuit at High Frequency (MHz)

translating the range 80 to 100 V to the range 0 to 5 V can be done with a pair of resistors.

since the input is 80 to 100, and the output is 0 to 5,
you want to multiply the signal by 0.25 and subtract 20 V
this can be accomplished by careful implementation of a summing junction
using an op amp (if the power is available)

you'll need a good quality reference to source the 20V subtraction
and probably 0.1% resistors

what power is available to operate the circuits?

what temperature range will this operate under?
the larger the range, the more important the temperature coefficient becomes

i assume you want to digitize the signal
you wrote "the highest resolution" - 10 bit? 12 bit? 24 bit?
how many bits does your microcontroller A to D converter have?

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3. ## Re: Step Down Voltage Circuit at High Frequency (MHz) Originally Posted by wwfeldman translating the range 80 to 100 V to the range 0 to 5 V can be done with a pair of resistors.

since the input is 80 to 100, and the output is 0 to 5,
you want to multiply the signal by 0.25 and subtract 20 V
this can be accomplished by careful implementation of a summing junction
using an op amp (if the power is available)

you'll need a good quality reference to source the 20V subtraction
and probably 0.1% resistors

what power is available to operate the circuits?

what temperature range will this operate under?
the larger the range, the more important the temperature coefficient becomes

i assume you want to digitize the signal
you wrote "the highest resolution" - 10 bit? 12 bit? 24 bit?
how many bits does your microcontroller A to D converter have?
It is a RF application. Actually, when I say "high resolution" I mean that the sine wave between the range 0-5 V needs to be clear (no noise). Like I said, it is a RF application, the TI microcontroller receives an analog signal, I do not need to pay attention on A/D converter or something like this. So do you think that a par of resistors can be a possible solution?  Reply With Quote

4. ## Re: Step Down Voltage Circuit at High Frequency (MHz)

Hi,

The very most scope probes use resistors as voltage dividers...plus a variable capacitor to adjust for high frequency.

Noise is one thing... here resistors are not bad.
Distortion is the other thing, here the resistors may be better compared with transformers.

I vote for a resistive divider.

Klaus

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5. ## Re: Step Down Voltage Circuit at High Frequency (MHz) Originally Posted by EltonBrasil highest resolution
An expanded scale meter is handy to help achieve this. It does not start to read upscale until the source reaches a threshold voltage. Install a zener diode inline with your source. Thus an 80V zener subtracts 80 from your 80-100V range. Creating 0 to 20V output.
Then divide it down with a resistor network so you get output under 5V. - - - Updated - - -

If an 80V zener diode is unavailable, it can be assembled from two or more zeners/ led's/ diodes.

- - - Updated - - -

Or suppose you have a 15V zener diode, and can accept a 1-4.4 V output range. The job can be done by putting the resistor divider first, and finagling with resistor values: 1 members found this post helpful.  Reply With Quote

6. ## Re: Step Down Voltage Circuit at High Frequency (MHz) Originally Posted by BradtheRad An expanded scale meter is handy to help achieve this. It does not start to read upscale until the source reaches a threshold voltage. Install a zener diode inline with your source. Thus an 80V zener subtracts 80 from your 80-100V range. Creating 0 to 20V output.
Then divide it down with a resistor network so you get output under 5V. - - - Updated - - -

If an 80V zener diode is unavailable, it can be assembled from two or more zeners/ led's/ diodes.

- - - Updated - - -

Or suppose you have a 15V zener diode, and can accept a 1-4.4 V output range. The job can be done by putting the resistor divider first, and finagling with resistor values: Looks very interesting this solution. Is it enough for high frequency (13.56 MHz)? Thanks much for your support.  Reply With Quote

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7. ## Re: Step Down Voltage Circuit at High Frequency (MHz)

Previous threads of yours suggests a RFID application (small band, AC only). In this case a pure capacitive divider could be used as well.

The best solution would be chosen depending on the source and load impedance, which isn't yet known.

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8. ## Re: Step Down Voltage Circuit at High Frequency (MHz) Originally Posted by EltonBrasil Looks very interesting this solution. Is it enough for high frequency (13.56 MHz)? Thanks much for your support.
My simulation portrays a concept although it may require changes to adapt it to your project. Example, I don't know whether every zener is able to respond properly to 13 MHz.

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9. ## Re: Step Down Voltage Circuit at High Frequency (MHz)

Hi,

I agree.
Standard zeners suffer from high parallel capacitance and thus I doubt they are useful for 13MHz applications.

Klaus

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10. ## Re: Step Down Voltage Circuit at High Frequency (MHz)

LED voltage drop is not constant. Putting them in series will yield a changing voltage drop.

resistor divider with capacitor for frequency or capacitor divider are best bet

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11. ## Re: Step Down Voltage Circuit at High Frequency (MHz)

Is this for extracting the data bits in an RFID reader for passive RFID tags?
In that case I think you only are interested in the changes in peak voltage.
An envelope detector with AC coupling to the microcontroller is probably better than scaling down the voltage to 5V. The changes in the peak voltage can be very small.

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12. ## Re: Step Down Voltage Circuit at High Frequency (MHz) Originally Posted by std_match Is this for extracting the data bits in an RFID reader for passive RFID tags?
In that case I think you only are interested in the changes in peak voltage.
An envelope detector with AC coupling to the microcontroller is probably better than scaling down the voltage to 5V. The changes in the peak voltage can be very small.
Yeah, I need to identify if there is a tag in the field or not. I have a TI reader and signal back from the antenna needs to be less than 5V and clear (no noise).  Reply With Quote

13. ## Re: Step Down Voltage Circuit at High Frequency (MHz)

Normally the reader is detecting tag presence by decoding the load modulation. It's very sensitive by operating in sync with the carrier.

Why do you want to detect tag load modulation on your own, not utilizing the reader receiver channel? To process the coil voltage with an uP ADC, you need to rectify the RF.

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