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Transistor based amplifier interface to ADC

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No, I connect the ground terminal to it. And there is a notable issue with my adc, when I give a FM signal with MMC40406, DAC will show the original signal but for CD4046, DAC will show distorted signal. What can be the reason .IS there any other ways to bias the ADC?
 

As I understand it:

A) When you connect the signal generator directly to the ADC input, the output signal from the DAC is fine.
B) When you connect the signal generator to the ADC input through a buffer, the output signal from the DAC is distorted.

What does the input signal at the ADC input look like in each case? There must be some difference. If the input was the same, the output would be the same.
 
There is a small voltage shift between the two signals due to AC coupling,nothing else.
adc_dac2.jpg
(Blue -original signal , Yellow -buffered signal)
 
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There is a small voltage shift between the two signals
Are you sure?
Under the traces, it says Vmax=2.8V, Vmin=1.82V for the yellow, and Vmax=935mV, Vmin=88mV(or 38mV?) for the blue.
That's a big difference.
 

The datasheet for the AD9280 ADC shows that its input bias current is zero but increases to 22uA when the input is 2V higher or lower than its bias voltage.
 
Thanks, Audioguru for your reply . So what kind of configuration is it accept. Dose it support the following arrangement?
emfout_cct.jpg
Here is the enlargement version of my ADC schematic.
enlage.jpg
 

I think you must set an upper voltage on pin REF TF and a lower voltage on pin REF BF by using the reference voltage built into the IC.

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An emitter-follower biased at half the 5V supply with a 1k emitter resistor cannot drive a load as low as two series 1k resistors across the supply.
Here is a simulation and the input signal is only 1V peak:
 

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  • emitter follower transistor.png
    emitter follower transistor.png
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I think you must set an upper voltage on pin REF TF and a lower voltage on pin REF BF by using the reference voltage built into the IC.
I am using this in a development board so no need to give the reference voltage to IC because the development board it self has a configuration as follows.
adc_dac.jpg
What are the other things I need to do in order to get the original signal from DAC?
 

I notice another different thing: My development board has a voltage display function through ADC. When I connect the ADC to 5v it display 5v ,when connect it to a ground it display 0V.When I create a voltage divider using two series 1k resistors and measure the value it display 4.5V . Is it due to input impedance of the source or any other reason?
 

Brober, Thanks for your reply. I checked that the ground is ok.
 

When I connect the ADC to 5v it display 5v ,when connect it to a ground it display 0V.When I create a voltage divider using two series 1k resistors and measure the value it display 4.5V
Are you sure you used two 1K resistors, not a 1K and a 10K?
What is the voltage at the ADC input when you connect it to the voltage divider? It should be 2.5V.
 

Analog input can be faulty it has 4.5V without 1k resistors?
 

godfreyl, as you told voltage at the ADC input is 2.5V. But ADC value shows 4.5V. What should be the minimum current input to the AD9280 IC? Datasheet shows that the typical input resistance 4.2k to 10k what is that? AS I went through the data sheet I found that the input resistance of the AIN varies due to the given configurations.What is the configuration related My ADC? I am not very good at referring the datasheet so can any body help me to solve my problem.
 
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