Calculate input power of ADC

Hello SebMay, 

Are you asking for the input so as not to saturate the ADC? You should be fine if you stick to the input voltage on Table 2.

Please correct me if I am completely misunderstanding the intent of your question!

Thanks, 

Kris

Hello krisf,

not quite, I am looking for the input impedance of the ADC, because I want to calculate the input power of my signal.

Here is an example:

Sinusoidal inuput of 0.25 Vpp, LNA input impedance = 200 Ohm, Vref = 1V

--> Input power = -14 dBm

channel gain = 16 dB

--> Power at the ADC = 2 dBm

According to your table, this is the maximum input voltage at 16 dB gain, so the ADC would sample a signal with 2 Vpp (1V reference Voltage)

Then the input impedance of the ADC can be calculated to be ~ 315 Ohm

Is this correct? Can I use 315 Ohm to convert any sampled signal to power, if I don't know amplitude or power at the input ?

The input impedance of the ADC changes to 3150 Ohm if I use 2kOhm as input?

Thank you!

Can anyone help me?

Hi SebMay,

Thanks for your interest in AD8283. I'll try to find the person who knows about the AD8283 to help with your questions.

For clarification, are you asking about the impedance and power at the internal ADC inputs, circled in the diagram below?

If so, why do you need signal power information at this location?

Thank you.

Doug

Yes, that is what I am looking for.

I want to calculate the power of a signal, which is sampled by the ADC, and compare it to the theoretical receive power I get from the transmission equation. This way, I can characterize the losses and levels in my receiver.

Thanks!

Hello Dougl,

any news?

Thanks!

HI SebMay,

I'm sorry for the slow response. I've asked for AD8283 support person for help with this, but they must be busy at the moment. I'll check again.

I'm sorry that I do not know the details of AD8283.

To characterize the loss and levels of your receiver, can you use the amplitude information at the inputs of the AD8283 (LNA inputs)?

Thank you.

Doug

Hello Dougl,

Unfortunately I don't have access to the input pins of the A8283 chip on the PCB. It would be very difficult to measure the input voltage at the chip input without partially damaging the PCB.

Thanks!

Hi SebMay,

The input full-scale voltage of the ADC within the AD8283 is 2Vpp_differential, according to the datasheet. This means that a maximum positive output code corresponds to +1V and a minimum negative code corresponds to -1V. If you are capturing the ADC output codes produced by your test signal, this is a way of getting a close representation of the voltage at the ADC inputs.

Does this help with achieving your goal?

krisfdo you have any other suggestions for SebMay?

Thank you.

Doug

Hi SebMay, 

The input seen at the ADC would be an unbuffered switched capacitance input. The impedance would be dependent on the sample rate - so it would change depending on what frequency it is being sampled at or how many channels you are using. 

I did make a few measurements a while back for 2 and 3 channels, and the differential input impedance varied from 5 to 8 kohms. However, as I mentioned that it is dependent on the sampling frequency, then this will change depending on your configuration.

I do think that the information at the LNA inputs would be much more predictable. 

I am not sure whether this gets you closer to what you are trying to calculate. Let me know. 

Thanks, 

Kris