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Analog Discovery 2 waveform output impedance


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I tried measuring the output impedance of the function generator by hooking it up to the scope with and without a 100Ω load resistor in parallel.  The voltage drop indicated to me that the output impedance was around 16Ω, not 0Ω (which is not really very feasible).  Of course, what I was probably measuring was the output current limitation of the AD8067 op amp (rated at 30mA for 60dB spurious-free dynamic range, or 105 mA short-circuit current).

It is probably more useful to think of the output as a voltage source (0Ω impedance) with a current limit than as having an output impedance.

I've posted a plot and some measurements at 

https://gasstationwithoutpumps.wordpress.com/2016/12/12/fet-i-vs-v-with-analog-discovery-2/

Edited by gasstationwithoutpumps
added link to plots
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Reply to the followup question:

When the resistance is set too low for the maximum current rating of the AWG, the voltage will drop until the current is compliant, correct? Even though the voltage drops the “Use as Reference” being checked means that the signal would still show as if it wasn’t being attenuated which is causing the confusion. Is that right?
Could you provide a screenshot of what happens when you uncheck that box? 

Of course, event with AWG zero Ohm output the voltage drops when the maximum current is reached. This current depends on the voltage level.
The zero Ohm output ensures a predictable output voltage, being in recommended output conditions, <10mA for AD/AD2
The output voltage of traditional AWGs has a certain impedance, so the output voltage always drops relative to the added load. With the "BNC Adapter Board for AD" you can select similar 50 Ohm output impedance or you can put a series resistor when working with the wires.

Having the "Use as Reference" selected under Channel 1 settings, only Channel 1 shows for Vpeak and Vrms the real value, for Gain and dB relative to the specified Amplitude. The magnitude shown for the other channels is relative to the measurement of Channel 1.
This is done to show the characteristics of the analyzed filter circuit, eliminating possible Wavegen signal attenuations at high frequencies or excitation signal current limitations.
This features is also useful to analyze sections of multistage filter, by connecting Scope Channel 1 to an intermediate section.
(In the next software version this option is renamed to "Relative to Channel 1" and moved to Magnitude settings.)

In this experiment AWG1/C1 is attenuated. C2 shows the absolute value and R1 the relative value to C1.

i1.png.4644985df220493e37ee59f9dc5b48e3.png

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Hi, this is an old post, but I am experiencing a related issue with the AD2 now. I am using AWG1 to generate a sine wave 1V Amplitude (2Vpp), f=100Hz and measure the output across a resistor, R=461 Ohm (measured with a DMM). The output of WG1 across the resistor is being measured with Ch2, on a breadboard. I am seeing a significant attenuation, as read on Ch2, instead of the expected 1V amplitude sine wave: I am reading ~646 mV amplitude.

If I increase the resistance value the output gets closer to the expected 1V. With R = 1MOhm, I do see 1V amplitude, as expected, same as by just measuring the output of WG1 on Ch2 with nothing in between (R -> Inf).

I cannot make sense of this, since it should not be exceeding the 10mA output of WG1: 1V / 461 Ohm ~= 2.17 mA. There is nothing else attached to this trivial circuit: just WG1 applied across the resistor and read with CH2 of the AD2. I am using BNC probes for the generator and for the measurement. I get the same identical result with 1x or 10x setting of Ch2 probe (which has been compensated).

Am I missing something obvious? Thank you in advance.

EDIT: I have performed additional tests and found that the problem disappears if I use the fly-wires instead of the BNC probes. It seems that the output of the AWG1 through the probe "sees" a series resistance of about 250 Ohm. I could not find documentation of this, in fact the documentation states that the output of the BNC module AWG's can be 0 Ohm or 50 Ohm only. I had it set to 0 Ohm source impedance. I would like to understand what is going on, if anyone knows.

Edited by christianb
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Hi @christianb

You are probably using oscilloscope probe for AWG (WG,Wavegen) and the voltage drops on this.
The oscilloscope probes at 1X have resistance of 100-230 Ohms to reduce reflections.
This is negligible when used with 1M Scope input impedance but it is considerable when used for signal source.

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19 hours ago, attila said:

Hi @christianb

You are probably using oscilloscope probe for AWG (WG,Wavegen) and the voltage drops on this.
The oscilloscope probes at 1X have resistance of 100-230 Ohms to reduce reflections.
This is negligible when used with 1M Scope input impedance but it is considerable when used for signal source.

Yes, that is the problem. The resistance of the probe was not reported in the documentation or datasheet. Thanks for clarifying.

One problem is that when using the BNC adapter the AWG from the fly-wires are disabled and cannot be used. Also, nowhere it says or suggest using a different type of lead. I would invite Digilent to update the documentation to make that clear.

Thank you.

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