I have to develop a system that samples 36 voltages synchronously. I've decided to use 3 AD9249EVM (FMC connector version).Those voltages comes from antialiasing filters after current measurements through measuring resistors. Which is the input voltage range of the board? If I understood correctly input signal of the board must be single ended in +-1V range: chip datasheet states 0 to 2 Vpp maximum ratings but in the schematics of the board I can see a centered tapped transformer that adds a common mode voltage.Firstly, I would like to understand if what I stated is correct, then I would like to know if there is any protection if the input voltage exceeds +-1V range, and if not, what I could do to prevent the ADC from damage. I was thinking about a voltage follower after each filter, with the opamp powered with +-1 voltage, so that the output should be limited by the supply voltage.Thank you in advance,Fabio
Thank you for considering the AD9249.
The input full scale range of the AD9249 is 2V peak-to-peak DIFFERENTIAL. At 2V peak-to-peak differential, each analog input swings 1V peak-to-peak. The input common mode voltage of the AD9249 in nominally 0.9V. So the full scale input voltage with respect to ground has each input swinging +/-0.5V above and below 0.9V. This results in 2V peak-to-peak differential and is shown in the figure below.
So, as far as I know what is stated in the datasheet is correct.
Input full-scale voltage is different from the maximum allowable voltage without damage to the part. As you can see from the figure above, this input full-scale scenario has the maximum peak voltage at 1.4V with respect to ground and the minimum peak voltage at 0.4V with respect to ground. This has a safe margin from the absolute maximum ratings for the AD9249 inputs, which is shown in Table 6 in Rev0 of the AD9249 datasheet.
If there is a risk of exceeding the voltages with respect to ground shown in Table 6 in Rev0 of the AD9249 datasheet, then input protection should be considered. The AD9249 has built in ESD protection, but it does not have overvoltage protection in the context you are asking about.
Input overvoltage protection usually takes the form of input clamp diodes, or designing an amplifier based front end that by design does not exceed the absolute maximum ratings, kind of like you mentioned except that you need to keep in mind the negative differential voltage is not the same as negative voltage with respect to ground.
Also, please keep in mind that all these input protection solutions will add distortion and/or noise.
Thank you for the answer Doug,The differential input isn't only for the AD9249 itself? In the EVM board there's just one connector per channel, isn't the differential signal generated inside the EVM board from the single-ended input?
And if yes, which is the single-ended voltage range at EVM input that guarantee 0-2 Vpp differential at the chip input?Thank you in advance, Fabio
The AD9249 requires a differential signal. The single-ended signal applied to the board is converted to differential by the transformer/baluns on the board.
If you need to GUARANTEE that you do not exceed 2Vpp differential at the ADC inputs, do not exceed 2Vpp single-ended at the board connector. There is some insertion loss from the baluns that will attenuate the signal slightly. According to the MACOM MABA-007159-000000 datasheet, it has a typical insertion loss of 0.32dB. Two of them in series results in 0.64dB insertion loss. If you do the arithmetic, 0.64dB insertion loss will result in 2.15Vpp single-ended at the balun inputs being about 2Vpp differential at the balun outputs. The insertion loss number in the MACOM datasheet is typical and can vary. So if you want an absolute guarantee to not exceed 2Vpp differential at the ADC inputs, do not exceed 2Vpp single ended at the board connector (which is directly connected to the balun inputs).