Can a instek sfg-1003 sink current? - Page 1
Forum section: electronics / testgear

author: Bugman53
author_rank: Newbie
timestamp: November 04, 2013, 01:10:41 am
content: I picked up a Instek sfg-1003 frequency generator to do some testing with a 5v square wave signal. The problem is the circuit has a 5 volt pull up resistor and a hall effect sensor triggers to ground to make the square wave. I noticed that on the generator it says do not supply voltage to the terminals. The circuit only supplies around 7mA of current to sink. Will it work or will I need to do something external to make the signal?Can I just put a resistor to ground to cancel out of the current or will it sink it itself?

author: kxenos
author_rank: Frequent Contributor
timestamp: November 04, 2013, 01:48:58 am
content: Why would you want to connect a signal generator to the output of a hall sensor? I don't get it. Is it possible you need a frequency counter?

author: Bugman53
author_rank: Newbie
timestamp: November 04, 2013, 02:14:33 am
content: The original signal came from a hall sensor connected to the circuit that has a pull up resistor and the hall sensor provides a ground to the circuit.What I and trying to do is replace the hall sensor with the generator on the bench. So I need to sink the current coming from the circuit.The circuit is a Speedometer and I am trying to do bench testing so I don't have to be going down the road trying to measure things.

author: kxenos
author_rank: Frequent Contributor
timestamp: November 04, 2013, 02:19:46 am
content: So you probably need to replace the hall sensor with a N MOSFET like for example a 2N7000. (Drain to pull up resistor, Source to ground and Gate to the input of the signal generator). That will simulate the function of the hall switch you have.

author: Bugman53
author_rank: Newbie
timestamp: November 04, 2013, 02:41:56 am
content: Thanks! that should be easy enough to build.

author: Rufus
author_rank: Super Contributor
timestamp: November 04, 2013, 03:38:21 am
content: Quote from: Bugman53 on November 04, 2013, 01:10:41 amYou don't really need to do anything. The generator will have 50 ohms output impedance. If you drive 0v into a pull up resistor to +5v that can source 7mA the generator output will be 350mV not 0v. That will likely be low enough and if not you can just set the generator output to swing a little -ve to compensate.

author: Bugman53
author_rank: Newbie
timestamp: November 04, 2013, 05:36:27 am
content: That is even better. Ill go test it. I was just worried it would somehow hurt the generator. As long as it swings lower than 1volt it should be fine.

author: Bugman53
author_rank: Newbie
timestamp: November 04, 2013, 05:48:55 am
content: I just hooked it up and it works great by itself! thanks!

author: AG6QR
author_rank: Frequent Contributor
timestamp: November 04, 2013, 10:24:16 pm
content: Hope I'm not coming in here too late. Sounds like you're in good shape, but for completeness sake...The SFG-1003 has two outputs: First is the normal output, with selectable waveform, adjustable voltage, and 50 ohm impedance. The second is the TTL output, which is always at TTL levels. The manual specifies that the TTL output has a fanout of at least 20 TTL loads. At the low state, a TTL output must have the ability to sink 1.6mA from each connected input. So the TTL output should be able to sink at least 20 * 1.6 mA, or 32mA, when it goes low. That should be more than enough for your purposes. FWIW, TTL outputs normally make better current sinks than current sources. That explains why, when working with TTL, you typically drive LEDs between Vcc and a TTL output, lighting up when the output goes low.For future reference, if the TTL output isn't adequate on its own, you could use the signal generator's TTL output to drive one or more TTL buffers, and use the buffer(s) to drive some other circuitry. An advantage to doing this is that if you make a mistake, you're likely to burn out the TTL buffer on your breadboard instead of the output circuit of your SFG-1003. Alternatively, you could use the signal generator to drive a FET, as already suggested.

author: Bugman53
author_rank: Newbie
timestamp: November 05, 2013, 04:55:37 am
content: I made up a board with banana plugs using a n mosfet to trigger to ground. It works great, I also added a 7805 powered by a 9 volt battery so I can add a pull up resistor to the mix if I need to for the output side to my oscilloscope. Works great and emulates the speed sensor perfectly.

author: Bugman53
author_rank: Newbie
timestamp: November 05, 2013, 04:56:31 am
content: Quote from: AG6QR on November 04, 2013, 10:24:16 pmThanks! I could not find any documentation as to how much it would sink