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Voltage gain of DTMF decoder

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dramrattan

New Member
Hi,

The attached DTMF decoder configuration was taken from the datasheet of the MT8870 DTMF decoder but i'm seeing that in order to calculate the gain, the gain formula have a S...What is this S?... Do i just use the gain as Rf on R? The equation is on page 9 of the attached document.

Regards.
 

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Mikebits

Well-Known Member
Ignore the S stuff, Just use the Rf/Ri formula. Close enough.
 

dramrattan

New Member
Voltage Gain

Lol...Oh well i'm seeing the voltage gain on page 5 for the differential configuration on the MItel datasheet is R5 on R which is the same as Rf on R in the testing circuit i attached earlier so that should be good...Thus it should be 150K/(56K+56K) which is approximately 1 so i don't see the sense using the differential configuration with a gain of almost 1 when there is a single ended input configuration with unity gain available. Unless the DTMF decoder is that sensitive to changes :confused:
 
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Mikebits

Well-Known Member
Differential has the benefit of noise reduction. The + noise on terminal a cancels - noise on terminal b. Does that make sense?
 

dramrattan

New Member
Diff. Configuration

Ohhhh yea makes sense...think i read that later down in the Application note...thanks for the refresh of memory...:)
 

ericgibbs

Well-Known Member
Most Helpful Member
hi dramrattan,
Consider what effect the Source impedance has upon the gain, its a nominal 600R.

Also a single ended input will give no common mode rejection, whereas a differential input will.
 

dramrattan

New Member
Diff. Input Config

Yea it was mentioned that the zero formed on the '+' when Ra is choosen os canceled by the pole formed by calculating Rb:

Rb = Ra X Rf/(Ra+Rf)

I can't remember what is a POLE and what is a ZERO so i trying to figure that out for the while
 

Mikebits

Well-Known Member
hi dramrattan,


Also a single ended input will give no common mode rejection, whereas a differential input will.
That is what I was trying to say... :eek:
 

ericgibbs

Well-Known Member
Most Helpful Member
Thanks eric gibbs...makes sense
hi dramrattan,
This is your cct with a gain of 2, [6dB], I have used a single ended input for demo.

The 3dB point is about 70Hz.
 

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ericgibbs

Well-Known Member
Most Helpful Member
Hi,

What did you use to simulate the circuit?...How do yo know the gain is 2 from that graph? :eek:
LTSpice.

The gain 6dB, look at the left Y axis of the graph at the 0bB line, the flat portion of the gain curve is on the +6dB line... OK.?
 

Mikebits

Well-Known Member
He might be asking, how do you decide on what the gain should be set to.
 
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