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Voltage offset

I want to read a voltage on a ADC that only accept 0-5V. My input
signal is 12Vp-p (-6 to 6). I simulate this circuit and it work. I
have 0 to 4.45V on Vout. My question is, do this circuit can easely
induce noise in my VCC. Or this only depend on how I will layout my
component on the PCB? Using a op-amp in differential is more safe than
this? My signal is 20 KHz.


(Use courrier new)

R1 - 1.2K
-----------/\/\/\/\----------------o Vout (go to ADC)
| |
| |
| |
| \
------ /
|sin(x)| \ R2 - 1K
------ /
| \
| /
| |
| |
| |
| ------- 5V
| ---
| |
| |
|-----------------------------
 
If you use a good low noise reference source to generate the 5V
offset, the additional noise from the off-setting source is unlikely
to be a problem. If you use a 5V rail as your offset source you could
well be in trouble.

Pardon me, but I don't understand the term "rail", do you mean my
power supply source? If so, yes my 5V *reference* is directly from my
main power
supply source.

From the thow reply a see, once said it's ok and you said I can be in
trouble!!!
What's the real story?

I have design a circuit using a op-amp to obtain the same result, but
op-amp is *expensive* compare
to the above circuit, but if a running after trouble with this design,
I will go for an op-amp.
 
M

Mook Johnson

Jan 1, 1970
0
I want to read a voltage on a ADC that only accept 0-5V. My input
signal is 12Vp-p (-6 to 6). I simulate this circuit and it work. I
have 0 to 4.45V on Vout. My question is, do this circuit can easely
induce noise in my VCC. Or this only depend on how I will layout my
component on the PCB? Using a op-amp in differential is more safe than
this? My signal is 20 KHz.


(Use courrier new)

R1 - 1.2K
-----------/\/\/\/\----------------o Vout (go to ADC)
| |
| |
| |
| \
------ /
|sin(x)| \ R2 - 1K
------ /
| \
| /
| |
| |
| |
| ------- 5V
| ---
| |
| |
|-----------------------------


I would highly suggest doing the attenuation/level shift using a good, high
bandwidth opamp to drive the ADC input. The ADC sucks a narrow, high peak
current when it samples and a 500ohm source impedance will surely wiggle in
this condition. A good high bandwidth opamp (say an AD8031) would make a
good buffer for the ADC input. Running the opamp off 0-5V will also protect
the ADC from damage if your sine input signal gets zapped or goes higher
than the expected voltage.
 
F

Fred Bloggs

Jan 1, 1970
0
Pardon me, but I don't understand the term "rail", do you mean my
power supply source? If so, yes my 5V *reference* is directly from my
main power
supply source.

From the thow reply a see, once said it's ok and you said I can be in
trouble!!!
What's the real story?

I have design a circuit using a op-amp to obtain the same result, but
op-amp is *expensive* compare
to the above circuit, but if a running after trouble with this design,
I will go for an op-amp.

This is about the same as your circuit but greatly attenuates the 5V
noise to be expected in a digital system...
View in a fixed-width font such
as Courier.
 
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