I am trying to design a circuit to get an amplified signal out of an OPT101 photodiode.

The photodiode contains an amplifier (and there is an additional opamp for further signal amplification on the board) that can be supplied with +/-15V, so I used an ITA0515S voltage regulator (VR1 in the schematic) to get these voltages from a +5V supply.

When I fully assemble the circuit, the voltages on the output rails of the voltage regulator drops to -2 and ~0 V. I am unsure if it is just shoddy workmanship on my side or if I built in some kind of short somewhere which creates this. Can somebody look over the schematic and tell me where I messed up?

Sorry that it is not all standard symbols - using a student Altium Designer license with the parts importer to get the footprints and that often only puts up boxes with pins on the schematic.

opt101 circuit

  • \$\begingroup\$ What is the input voltage to the regulator when the output voltage drops? \$\endgroup\$ Commented Nov 11, 2023 at 10:56
  • \$\begingroup\$ Should -Vin and Common be connected? \$\endgroup\$
    – MiNiMe
    Commented Nov 11, 2023 at 11:11
  • \$\begingroup\$ @MiNiMe As I use no neg. input voltage, I saw no reason not to? \$\endgroup\$
    – Sacharon
    Commented Nov 12, 2023 at 12:39

1 Answer 1


The two sources of MFT1 MOSFET are connected to nothing, except to each other. You can't expect to have any current out of the drain D2 which is supposed to feed +VIN on VR1.

  • \$\begingroup\$ Hey, this configuration was actually taken from the datasheet of the protection IC datasheet as back-to-back n-channel config for voltage protection. But as you are asking I feel like I missed some basic mosfet concept - can you compare with the DS (page 8, fig. 6) and tell me if I misunderstood it? I used a slightly different current mosfet (FDC8602 from onsemi), but same pin layout, because original was not avail. \$\endgroup\$
    – Sacharon
    Commented Nov 12, 2023 at 12:38
  • \$\begingroup\$ @Sacharon When G1 and G2 of MFT1 are high, the current can flow in both directions. When they are low the current can't flow in neither direction. The current has to pass through the body diode. It also passes through the body diode of the P-MOSFET. Check if these body diodes can support the required current. If not add external diodes. \$\endgroup\$
    – Fredled
    Commented Nov 14, 2023 at 2:29
  • \$\begingroup\$ Yes, exactly, so there should be no issue, total current of the whole circuit after is probably in the 100mA range (roughly guessed)... However I am still unsure if there is any issue with anything BEHIND the VR1. Do you see anything? \$\endgroup\$
    – Sacharon
    Commented Nov 15, 2023 at 15:49
  • \$\begingroup\$ I think that COMMON should be completely disconnected from GND, and be used as ground reference for all the circuitry powered by the VR1 output. Also bear in mind that max output current is limited to 30mA. If the circuit uses more than that it will affect voltage. xppower.com/portals/0/pdfs/SF_ITA.pdf \$\endgroup\$
    – Fredled
    Commented Nov 15, 2023 at 22:16

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.