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    0-1A Bidirectional Current Source

    I'm trying to design a bidirectional current source for the purpose of measuring very small (0.0001 Ohm) resistances. I've come across this circuit: I can see that the transistors are there to 'boost' the current but I don't understand the purpose of the 1K resistors and the capacitor. I...
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    A Simple Ohmmeter - Design Dillema

    So after looking at both PNPs and P-Channel MOSFETs, I think I'll go with the MOSFET variation. But I've run into another problem here and am looking for advice. The cable that I'm measuring can be upto 500m long. This in itself isn't a problem but the cable picks up all sorts of noise. There...
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    A Simple Ohmmeter - Design Dillema

    Thanks uart and averagesupernova. Im going to try and build this circuit a bit before I commit to the design. One question about the MOSFET: you state I need one with low threshold voltage and that I completely understand. However, normally, a MOSFET's source is tied to ground and the gate...
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    A Simple Ohmmeter - Design Dillema

    Here we go... sorry I didn't supply a schematic earlier. Note, Q1 is now a generic PNP transistor. In the LTspice sim. This circuit works when there is a zero ohm test resistor but when there is an open circuit present at the test resistor the current flows through R1, through the emitter...
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    A Simple Ohmmeter - Design Dillema

    uart and averagesupernova, Thank you very much for that explanation. I simulated the circuit in LTSpice and you were right - the ckt doesn't work. I think I missed the fact that an NPN would cause the feedback to be positive instead of negative. One question, however, does this handle the case...
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    A Simple Ohmmeter - Design Dillema

    Got it: Replace Q1 with a darlington. Makes sense. Your second point, regarding changing the swapping the inputs of the op-amp confuses me. This is my first analogue circuit (I have designed and implemented digital circuits before). My question is, why would the circuit be better if I simply...
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    A Simple Ohmmeter - Design Dillema

    I've been design a simple ohm-meter which can measure resistances in the range 0.001 to 100 Ohms. I am aiming for an accuracy of 3%. Here's the circuit (so far): The circuit is basically a programmable current source which can source a current of 100, 10 or 1 mA. At junctions Va and Vb...
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    Terminating and Routing SPI

    I'm looking for some advice regarding how to terminate SPI signals which are going to an external PCB. I know that SPI isn't meant for board to board communication but in this case the connecting ribbon cable is short - about 6" max length. My frequency isn't high, just 2 MHz but I have...
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    Pulse Skipping - Calculating Frequency Limits

    Thanks! Sorry about the late reply - I didn't notice anyone replied. The input voltage is just 7V. Actually, I can make it anything I want as it's in my hands, so I'm going with 7V. The 5V and 3.3V rail have the highest current requirement (0.5A and 1A respectively). My initial estimates...
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    Pulse Skipping - Calculating Frequency Limits

    I'm trying to design my first switching regulator circuit. I require three voltage rails, 5V, 3.3V and 1.8V. But to get started, I'm just focusing on 1.8V. I've decided to use the LT3507 as it seems to be commonly available here. It also has 3 buck regulator outputs, so it's a one-chip solution...
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    External Power Supply or Put Regulator on Main PCB?

    Thanks. I've already found some that I like. One question I had though - is it worth it to integrate the power circuitry on the PCB? What is the disadvantage of keeping the power supply external? If I keep the PSU external, I don't need ventilation for the main circuitry and it can be...
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    External Power Supply or Put Regulator on Main PCB?

    Any regulator that you could recommend? I would prefer a easy to use one as this would be the first time I implement switchers!
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    External Power Supply or Put Regulator on Main PCB?

    I'm close to finishing my first 4-layer board. Initially, we felt that keeping the power supply as a separate unit would be better. This was because we wanted to avoid having heat on the main PCB - mostly because the area where the device is intended to be used is very hot and also quite dusty...
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    TVS Diodes for ESD Protection?

    I had a small question about ESD. I have implemented TVS diodes on my project at the input side. However, the output is still unprotected. It's a bit like this: CPLD 1 -> Logic-Level MOSFET -> WIRE UNDER TEST -> TVS DIODE -> Pull up -> CPLD 2. So the input side is protected from ESD...
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    Puzzled by MOSFET switch

    I have the following circuit wired up on a protoboard. The datasheet for the BSS138 MOSFET is here. I'm puzzled regarding what I'm seeing happen with this circuit - when I apply 3.3V at the gate resistor, the MOSFET turns on fully and I see 3mV at the output. This, of course, is expected...
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    Decoupling Caps' Via Position

    Here's the picture I promised, but it seems yungman already provided a solution :): As berkeman said, I don't have a high frequency requirement. So I feel 0.1uF would be OK. Regarding 47uF, I'm using an Altera CPLD and they recommend 0.01 to 0.1uF decoupling and 47uF to 100uF for bulk...
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    Decoupling Caps' Via Position

    Thanks for all the advice, guys! I'll stick with this layout but may have to deviate from it slightly. I'll describe the issue below. Yungman, the cap are on the same side as the chip. I was tempted to put it below, but felt the decoupling is most important. I also have 47uF caps about 2-3...
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    Decoupling Caps' Via Position

    I'm designing a 4-layer board with a couple of CPLDs and a uC. The top and bottom layer are for signals and the 2nd layer is ground and the 3rd layer is 3.3V. I'm placing a 0.1uF X7R Ceramic cap in 0402 package on every Vcc/Gnd pair. The thing that confuses me is that I've seen several ways...
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    TVS Diodes for ESD Protection?

    Thank you very much, yungman!
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    TVS Diodes for ESD Protection?

    After some reading, it seems I need determine the peak pulse current. But how do I estimate the voltage that will develop due to ESD? I believe I need to divide that by the impedence of the circuit. Would appreciate any pointers regarding this.
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    TVS Diodes for ESD Protection?

    OK, the cable connects to a MOSFET's drain and the other connects to a CPLD's pins. The output high voltage is just 3.3V. To illustrate: CPLD A's Output -> MOSFET -> Connector -> Wire Under Test -> Connector -> CPLD B's Input I think there should be ESD protection between CPLD B and the wire...
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    TVS Diodes for ESD Protection?

    I'm developing a basic cable continuity tester. Essentially, the circuit drives a test vector onto the cable and reads back the output vector. The output has MOSFETs for buffers/drivers but the input is connected directly to the ICs (CPLD, in my case). My concern is ESD. Unfortunately, I have...
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    Would a four layer board be beneficial for me?

    Yes, the current board that I'm designing is actually meant for a long-term test. The aim is to place four of these on the floor and use them alongside the old machines. Regarding, voltages. The CPLDs (Altera Max V) can only work at 5V but through a transistor I can of course switch 5V. But the...
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    Would a four layer board be beneficial for me?

    Another thought that occurred to me. What if I forget TVS diodes and pull ups and instead put another MOSFET on the input. And increase the testing voltage to 12, possibly even 24V. Would this help against interference? A bit like this: [Sorry about the badly drawn schematic. I had this on...
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    Would a four layer board be beneficial for me?

    Thank you berkeman. I hope RF interference doesn't become a problem for me. I have too many components on the board as it is! But I wanted to ask, what made you thought that RF won't be a problem for me? The wires themselves will make good antennas considering their length. Was it the lack of...
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    Would a four layer board be beneficial for me?

    Thank you, indeed the pull-ups are located at the input. In general, when does RF interference become an issue? Does it depend on the frequencies we're operating? I'm sure the length affects the things. I should add, we don't need to any certification for RF emission etc. because the units will...
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    Would a four layer board be beneficial for me?

    Nope, just wire-crimpers but they're on the floor below and I don't think they cause much of a fuss. Wires at switched at 3.3V but we can take this upto 5V if needed. As an aside, in general, would a stronger pull up help us in this case? I mean, if I put in a 1K pull up than does this mean...
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    Would a four layer board be beneficial for me?

    Thanks berkeman. Will check that out as well. I think TVS diodes also come in arrays, so that's promising as well. One more thing I'd like to ask - I asked this very question on another forum and was told I should add RF filtering as well. Would this be necessary? Incase the operational...
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    Would a four layer board be beneficial for me?

    Sorry to bump this thread again, but I had another question - should I be concerned about ESD damage? The wires can be quite long, about 10 meters. The output, as mentioned, has MOSFETs as buffers. However, the input side is completely naked with the exception of pull ups. Should I put in a ESD...
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    Would a four layer board be beneficial for me?

    Not a problem. :) I appreciate the responses! Thanks. I had a hunch it would be that, but I wanted a more experienced engineer's opinion on the matter before I went ahead and designed the thing.
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