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Hello,

I am trying to use the AD 7366-5 A/D converter in conjunction with a Spartan 3E FPGA. This circuit was designed many years ago and has been primarily used to determine current polarity in power circuits in addition to other tasks. The usual frequency of these sinusoidal current signals is 60Hz and the purpose is to generate a logic 1 when current is positive and a logic 0 when the current goes negative. For a 60Hz signal, the circuit has performed very well all these years.

However, when I try to perform the same polarity detection with a 20kHz sinusoidal signal, I can see an appreciable delay of about 2uSec between the zero crossing of the analog signal and the time the digital signal goes from Low to High(or vice versa). The serial clock to the ADC is at 25MHz.

1. Is this delay to be expected and is it reasonable ? What parameters on the data sheet help determine this delay ?

2. If the application demands that this delay be as low as possible, what other options are available ?

3. Although not exactly pertaining to the above described problem, what is the correlation between the 'Throughput rate' and the maximum frequency of the analog signal that can be measured ? For instance, the Throughput rate for AD7366-5 is listed as 500kSPS. Does this mean that the maximum frequency of the analog signal that can be measured with this chip is 250kHz according to the Nyquist theory ?

• With a 25  MHz clock, your conversion time is about 2 us.

(@48 MHz clock, throughput is 1.12 MHz)

To take a design that was designed to work at 60 Hz, doesn't mean

it will work at xx kHz.  Why don't you just use a fast comparator?

Harry

• @harryh Thank you for the reply. You are right in pointing out that a 60Hz design will not work at several kHz. However, given the throughput rate of the ADC, I assumed it would work at this higher frequency. I may be wrong and hence I asked the question 3.

A fast comparator is also a valid option. However, I was simply hoping to avoid any additional hardware if possible.

• The 2 us conversion time was always there, it's just that at 60 Hz, it was insignificant

and you didn't see it.........

Harry

• This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

Thank you,