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AD9446 and AD8139

Question asked by FG@LTXC on Jun 7, 2012
Latest reply on Jun 21, 2012 by FG@LTXC

Hello,

 

I,m a young application engineer for LXT-Credence company.

 

I have design for a customer who wants to test infra red sensor, a loadboard.

As the sensor delivers an output analog signal, and contains 288x384 pixels, I decided to use an ADC to be able to capture all the results thanks to the Digital board of my tester (cause of memory depth needed-arround 11Mo, and the data analyze to do).

This sensor works at a frequency up to 10 MHz. We choose to test the device a 6MHz using an ADC driven by a Differential Amp (converting single ended signal to differential signal). The analog output of the sensor is from 1.0V up to 4.2V.

With this frequency and the analog range, I decided to choose the AD9446 as data converter.

To drive this ADC, I choose the AD8139 and I create the following :

 

 

The analog supplys of the AD8139 are +7V and -3.8V.

 

Here is the AD9446 used :

 

The clock frequency of the ADC is 60MHz, but I can only capture one data every 6MHz because of the output sensor settling time.

I currently have a standard deviation of 5.6mV for each pixel of the sensor with this application board (each pixel measured 100times). The real value of the standard deviation of pixel sensor is about 200uV.

 

To correlate with the real value of the sensor, I have to place a 22nF capacitor between the two ADC input, creating a 300KHz low pass filter.

 

My question is the following :

Do you think that the 60MHz ADC clock can generate as much noise on the ADCinputs?

What could be a good method to create a 6MHz low pass filter with a high rejection of the 60MHz (RC between inputs, RC between inputs and supply voltage, RLC only) ?

 

Do not hesitate if you do not understand some points.

Thanks on advance.

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