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Capacitor type and value for circuit

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quintessential

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hello, so i have this circuit(suggested by friends on this forum) that provides me a DC voltage supply superimposed on an AC voltage supply. The DC supply is great but i'm trying to get a frequency rersponse of the AC signal up to 10MHZ.The key is the capacitor in the circuit.It is what couples the AC and DC sources together but as you can see it creates a filter that slightly attenuates my AC signal.Basically i'm asking if anyone is savvy with capacitors and can offer a suggestion as to the type(electrolytic,ceramic.....) and value(10pF 100nF) that would give me the best and widest frequency response.
 

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kchriste

New Member
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What is the lowest AC signal you want to couple? If it is in the audio range and you want to go all the way up to 10Mhz, you can put some ceramic caps in parallel with a electrolytic to form a cap that will work across such a wide bandwidth (Say 1000pf, 0.01uf, 0.1uf and 47uf). Or use a tantalum cap, though you'd have to check the spec's to see if the particular one you have will work properly to 10Mhz.
 

quintessential

New Member
kchriste said:
What is the lowest AC signal you want to couple? If it is in the audio range and you want to go all the way up to 10Mhz, you can put some ceramic caps in parallel with a electrolytic to form a cap that will work across such a wide bandwidth (Say 1000pf, 0.01uf, 0.1uf and 47uf). Or use a tantalum cap, though you'd have to check the spec's to see if the particular one you have will work properly to 10Mhz.
i'm going to try the circuit with the lowest singe capacitance value tantalum cap i can find. you just confirmes past research that Tantalum caps work best for AC coupling and DC blocking type circuits.
then i'll try it with a capacitance network as you suggested.As for The mix of ceramic and electrolytic caps to form a wide bandwidth cap,does this work
a)due to their inherent nature
b)due to the range of cap values
c)both a) and b)
Hopefully something will come close to my target of flatband response from DC to 10mHz.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
quintessential said:
Hopefully something will come close to my target of flatband response from DC to 10mHz.
Making something flat from DC to 10MHz isn't trivial to start with - but I suggest you tell us EXACTLY what you're wanting to do, as opposed to how you think it should be done. As it stands I don't have the faintest idea what you're on about?.
 

kchriste

New Member
Forum Supporter
quintessential said:
As for The mix of ceramic and electrolytic caps to form a wide bandwidth cap,does this work
a)due to their inherent nature
b)due to the range of cap values
c)both a) and b)
Hopefully something will come close to my target of flatband response from DC to 10mHz.
The answer is A. Electrolytics don't work very well at high frequencies but ceramic caps do. Ceramic caps also have self resonance points due to lead inductance so adding different ones in parallel will tend to swamp this effect. But I wouldn't worry too much about it being totally flat anyway since you can compensate by adjusting the signal generator output while monitoring the ripple on the DC input to the regulator.
Nigel Goodwin said:
As it stands I don't have the faintest idea what you're on about?
It is related to this thread:
http://www.electro-tech-online.com/threads/what-op-amp-to-use.31477/
 
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