Analog Discovery 2, Waveform Generator Resistance

[latot]

Hi hi, I just want to confirm this, because if not, I can break it, when we use the waveform generator, if we connect the wave form with the ground, what would be the current?

I read in the specifications that the "input impedance" is 1Mohm, so if I have a sin function of 5V, the current would be 5/1000000, 0.005mA, or I'm interpreting bad this? I know can be a silly question, but I prefer to be sure about this.

Thx!

[attila]

Hi @latot

It is safe to connect Wavegen outputs to Scope inputs.

The Wavegen outputs have zero output impedance, drive of 10-50mA, at least 10mA at +/-5V and ~50mA around 0V. Have short protection between +/-5V but it is not recommended for long period of time since heat will be generated.
The Scope inputs are rated to +/-50V.

https://digilent.com/reference/test-and-measurement/analog-discovery-2/start

[latot]

Hi, I read the start, but is not like describing that, I don't want to connect the waveform generator to the scope, I want connect it to a solenoid for a fluxgate magnetometer, so, will be a connection from AWG to ground, I need to can keep the connection a long time, so heat is a good point. I need to calculate the impedance to minimize heat and maximize current, drive 10-50mA tells me there is resistance, or again, I'm confusing something, 0 impedance at output should cause high current.

[attila]

Hi @latot

The output impedance is zero because the PTC (fig19 R145) is in the feedback of the output amplifier:
https://digilent.com/reference/test-and-measurement/analog-discovery-2/hardware-design-guide#awg_out

[latot]

:O Thx!

What is the max current the waveform can have? I don't want to burn it D:

Or I can connect the waveform to a extenal ground, but in any case, I need to know the limit, or I can burn circuits inside.

[attila]

Hi @latot

There should be one ground, the external circuit/device GND should be the same as the AD2 GND and this is connected to the computer via the USB cable.

The Wavegen output is protected for shorting it the ground, to the other Wavegen or to the device's +/-5V supplies.

I've just measured the short to ground current of +55mA and -45mA, with slightly higher peak (~65mA) until the PTC kicks in.

[latot]

:O So its safe to connct directly to ground? who protect the circuit limiting the current, the wavegen or ground?

[attila]

Hi @latot

It is safe but not recommended.
The device won't be damaged but it is never recommended to push anything to the limits. The AWG output should be used as a voltage source up to 10mA. The protection PTC will heat up in considerable time, milli... seconds..., the output current will gradually drop and slightly depend on the ambient temperature. If the predictable current is important for your use-case use at least a limiting resistor.

Edited May 24, 2022 by attila

[latot]

Mmmmm, so what would be the right values to use?, not the max/limits, the max in right to use. 10mA? The ideal power to not heat the protection.

 

I'm thinking..., why the limit is in the current? usually the heat is proportional to the resistance, (V*I), without high voltage I should theoretically be able to get more current without more heat.

Edited May 24, 2022 by latot

[latot]

Hi @attilasomeone knows how to use safe the AWG?

[attila]

Hi @latot

Use a ~500 Ohm series resistor when connecting it to low impedance to limit the current to 10mA and you can safely generate +/-5V signals.

The device is protected and won't be damaged if you short the AWG output directly to anything between +/-5V, including GND. This is a protection and it is not recommended to be hold for long duration due to generated heat and unpredictable (temperature/time dependent) 45-65mA current.
In regards to you heat/power question: The PTC is in the feedback loop and when shorting the output to GND the output amp will push it above +/-5V, so more than 5V will drop on the PTC, the worst is when connected to +/-5V, the drop will be above 10V.