Logic analyzer input impedance

[algailani]

Hi,
I'm trying to measure the input impedance of my AD2 Logic analyzer and compare it to my new Digital Discovery impedance. My measurement is around 650 KΩ for the DD, however, I couldn't get any consistent result for the AD2 nor AD1. The input impedance varies between 30 MΩ and 70 MΩ. I used different techniques in my measurements, but with no success. I also looked the specs of D2/AD1 and couldn't find a figure for the Logic analyzer input impedance.

Can someone please, help  me find an answer or suggest a technique to measure the Logic analyzer input impedance?

Thanks

[jpeyron]

Hi @algailani,

Sorry about the delay in a response. Your thread is in the correct section to get the most experienced feedback for your question.I have tagged the most experienced engineer for your question( @attila ).

thank you,

Jon

[algailani]

Hello @jpeyron,

The methods that I used to measure the input impedance of the AD2/AD are as follows:

1. While powering the device, I used a FLUKE 289 (50 000 count meter) in ohm mode to measure the impedance between the digital channels and GND.

2. Applied a 3.1v from a battery to a Digital I/O pin and inserted the FLUKE in series between the batt and I/O pin in μA range to measure the current passing into the  I/O pin while the AD is running as a Logic analyzer. I used a battery to eliminate the noise from the switching power supply, in order to get clean and accurate readings as much as possible. 
input impedance = Batt Voltage / input current
Also, measured the PTC (used to protect the FPGA pins) inside the the AD, to verify that its value remains relatively constant during operation. I got readings around 221Ω from the PTC, which is consistent with the datasheet. 

3. Applied a low frequency of 50 Hz square wave (did sin wave as well) from the built in Wave Gen, with a series 100Ω resistor between the I/O pin and Wave Gen.
input impedance = WavGen voltage / (voltage across the 100Ω /100)
for this measurement, I set the FLUKE to AC+DC mode. Also viewed the voltage across the 100Ω (which is a function of the input current) and the WavGen output on the AD scope to get an idea about the input currents when changing the frequency/amplitude of the input. 

I hope this info is helpful to provide some answer.

Thanks,

Mohammed

[attila]

Hi @algailani

The Analog Discovery/AD2 DIO impedance when used as input is basically the Spartan 6 FPGA impedance.
https://reference.digilentinc.com/reference/instrumentation/analog-discovery-2/reference-manual#digital_io
https://www.xilinx.com/support/documentation/data_sheets/ds162.pdf

Here you can find information about the Digital Discovery DIN and DIO pins:
https://reference.digilentinc.com/reference/instrumentation/digital-discovery/reference-manual#input_dividers
https://reference.digilentinc.com/reference/instrumentation/digital-discovery/reference-manual#io_level_translators

[algailani]

I Spent some time looking into the Spartan-6 datasheet, but couldn't find useful information about the input impedance/resistance.

The reason for not having a consistent value for the input impedance is that the input current is not static, in other words it is always changing as if there was some capacitor in the input path. 

Can some one provide some explanation to this phenomena, or confirm that the input impedance is always changing?

Thanks