So I'm trying to set up an A/D interface to communicate with a high voltage power source. The power source has a 7 pin J2 programming interface, and says that a voltage of 0-5 volts applied to the programming pin will set the output voltage proportional to that input (HV output is 0-5 kV). I've got a LabJack A/D interface that supposedly puts out a 5 V potential, but when I plug it in, the HV is stubbornly mute.
Now I've gone about troubleshooting this and am still completely stumped. The HV has a pin with a 5V bias to be used for setting up a potentiometer, and when I connect this to the programming pin, I get the expected output (5 kV from the HV). I've also had limited success setting a function generator to scan the voltage, though it was a little buggy. I at first thought it might be a problem with the source impedance, and installed a voltage follower to boost the current. However, taking a look at it with an ammeter shows that the impedance on the programming pin is 5 kOhm, which should be plenty high (the impedance on the A/D source is given as 50 Ohm, though the ammeter says it's closer to 150).
I then figured I'd try to measure the current directly across the connection. When I did this, I predictably measured a 1 A (5 kOhm impedance) current across the programming pin. Everything seems to check out: the programming pin is biased 5 volts above ground (both devices are grounded to AC) and the same amount of current is flowing through the programming pin as it would if it was biased with the 5V pin: the only difference is the source of the 5V input. The only thing I can think of is to do some kind of impedance matching (too low source impedance?) but no one ever thinks to do that because the only people who care about such delicate impedance matching are audio engineers, and I personally have no idea how one would go about that, short of sticking a resistor in front of the source :/. It's safe to say the difficulty of this problem now exceeds my ability: anyone have any ideas?
Now I've gone about troubleshooting this and am still completely stumped. The HV has a pin with a 5V bias to be used for setting up a potentiometer, and when I connect this to the programming pin, I get the expected output (5 kV from the HV). I've also had limited success setting a function generator to scan the voltage, though it was a little buggy. I at first thought it might be a problem with the source impedance, and installed a voltage follower to boost the current. However, taking a look at it with an ammeter shows that the impedance on the programming pin is 5 kOhm, which should be plenty high (the impedance on the A/D source is given as 50 Ohm, though the ammeter says it's closer to 150).
I then figured I'd try to measure the current directly across the connection. When I did this, I predictably measured a 1 A (5 kOhm impedance) current across the programming pin. Everything seems to check out: the programming pin is biased 5 volts above ground (both devices are grounded to AC) and the same amount of current is flowing through the programming pin as it would if it was biased with the 5V pin: the only difference is the source of the 5V input. The only thing I can think of is to do some kind of impedance matching (too low source impedance?) but no one ever thinks to do that because the only people who care about such delicate impedance matching are audio engineers, and I personally have no idea how one would go about that, short of sticking a resistor in front of the source :/. It's safe to say the difficulty of this problem now exceeds my ability: anyone have any ideas?
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