Capacitor in a voltage divider used on a sine wave?

[Triode]

My coworker (not an EE) asked me (also not an EE) why his voltage divider isn't working. He's putting a 72MHz 3.3V peak to peak (ground to 3.3+) sine wave through a restive voltage divider. It's not making it through. Connecting an Oscope between the singal and ground it's just some noise. The source is a TLD3501, going rail to rail from a 3.3 volt source which shares the ground.

I'm no analog expert so I don't know why this is happening either. Checked and the resistors are carbon film not wire wound so there's not much inductance. Looked online for answers and all the schematics use a capacitor:

Google image search for "sine wave voltage divider" most schematics show a chain Vin-Capacitor-Vout-Resistor-Gnd, sometimes the resistor is second and the capacitor is second.

https://www.google.com/search?q=sine+wave+voltage+divider&es_sm=122&source=lnms&tbm=isch&sa=X&ved=0CAcQ_AUoAWoVChMI4dTHn66GxwIVwymmCh07UQ95&biw=1097&bih=568#imgrc=RYgkwlnR8QDImM:

example

The big question:
Is this capacitor-resistor divder the right way? If so why, and how do I calculate the capacitor and resistor values to use?

Thank you

 

[Triode]

Ok, I think the page that image is from has the answer.
http://www.planetarduino.org/?cat=427

 

[Tony Stewart]

series R+ shunt C is low pass, series C + shunt R is high pass and series LC is band pass in series with the right load and source.

 

[Triode]

That's how I understand it. I'm really perplexed as to why his resistive voltage divider isn't working though. I get that capacitors can attenuate a signal, but I don't see any reason resistors should just kill it like that.

 

[JimB]

He's putting a 72MHz 3.3V peak to peak (ground to 3.3+) sine wave through a restive voltage divider. It's not making it through.

A resistive voltage divider is basically two resistors in series, the input connects across both of the resistors, the output connects across one of the resistors.

A capacitor and a resistor are a filter.
Depending on which way the resistor and capacitor are connected, you will get a high pass filter or a low pass filter.

Connecting an Oscope between the singal and ground it's just some noise.

You do not state what the resistor values are. If the attenuation ratio is too high, all you will see is noise.
How is the scope connected to the divider?
You need to use a x10 divider probe.
If you use just a length of screened cable to connect the divider to the input of the scope, the capacitance of the cable may be enough to form a low pass filter in conjunction with the divider and kill the signal.

JimB

 

[Tony Stewart]

Not sure what you understand but a 0.1uF is almost a short circuit at 72MHz.
Notice the rolloff attenuation is -122 dB

 

[jjw]

What is TLD3501 ?

 

[ronv]

Ok, I think the page that image is from has the answer.
http://www.planetarduino.org/?cat=427

Yes, that page has the answer. Calculate the impedance of the capacitor at 75 Mhz. and insert that value into ohms law to see what the output will look like.

 

[audioguru]

Why are you feeding the very high frequency of 72MHz into the voltage divider designed for the very low frequency of only 50Hz?????
The capacitor value is WAY TOO HIGH for 72MHz and 30k ohms.

You forgot to say the attenuated level you want the output signal to be. For the 72MHz 3.3Vp-p signal to be reduced to 3.3uVp-p with the 30k resistor then the capacitor must be 0.074uF. 3.3uV is a very very low level.

 

[Triode]

The resistors were 10 kΩ and 12 kΩ. We were going for 1.8V.

What I meant was: I can understand how using a capacitor in a voltage divider for AC would work (though not with those values and this frequency) but I don't quite understand why our circuit, with two resistors, does not work.

 

[Triode]

What is TLD3501 ?

Sorry, I meant TLV3501
https://www.ti.com/product/tlv3501
He told me the number vocally, I must have heard the V as a D.

 

[jjw]

Can you post a diagram of your circuit.
How is the 72Mhz signal made?
Did you check the 72 Mhz signal before the resistive voltage divider?
High resistor values + some stray capacitance make a low pass filter.
For example 10 kohm + 10 pF has about 32dB attenuation at 72Mhz.

 

[AnalogKid]

Unless explicitly stated to be a non-inductive design, carbon film resistors are spiral cut and therefore have a significant inductance at 72 MHz. We still have not seen a schematic of your actual circuit, or a sketch of the test instrument connections. Without those, we're all just guessing.

ak

 

[tomizett]

I think jjw has your answer - if your scope probe has 15pF of tip capacatance it will look like 150ohm at 72MHz and your divider will have an attenuation of 36dB, which is to say, you'll only see 15% of what should be there without the scope attatched.