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open-hardware-voltmeter-01 Disclaimer If you order or use this you agree to and understand that the author is not liable for any injury or damage howsoever caused and that you use this PCB at your own risk. Thanks for helping keep SourceForge clean. I thought : why not to put an NOR gate 74x open hardware voltmeter 01 Roast Coffee Instead. Please enable JavaScript to view comments.

Is it difficult to do auto ranging for the 2 ranges? I found out how: it uses an analog front end that biases up 1. Voltages are sampled at both probes iirc and subtracted in the stm. Hmm, definitely worth a look also as training aid for new naive technicians we have so much fun with ;- Thanks for posting. Great for a hobby project, but if you are working with AC, or anything above a few mA, you are playing russian roulette, and loading more than one projectile.

I think this is a great project. Everything open source of course. Please be kind and respectful to help make the comments section excellent. Comment Policy.

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Report comment. Congratulation, nice job!!! We are currently also working on an open hardware version, stay tuned…. Leave a Reply Cancel reply.

The value should be as high as possible. This is the maximum current which has to be limited by a serial resistor. The maximum reverse voltage that can be applied is the power supply voltage and the maximum value of the diode's c ontinuous reverse voltage. The current which will flow through the resistor and the diode is:.

The limit current value must not exceed the maximum value of the diode's c ontinuous forward current and the resistor's power rating has to be selected based on the current and the resistance. Two values of the limiting resistors are given - 2K and 10K. Calculated values of noise and leakage voltage are shown in the last two rows in the table.

Even with 10K limiting resistor, the noise is low and can be ignored. However, the error due to current leakage of the whole front end which includes the reverse current of diodes and the input buffer can be significant. In the table, calculation shows that for pA total leakage which was my total error budget, the voltage error can be between 0.

On the 1V range, this makes 0. The small value of the limit resistor is preferably, but this will increase the current.

Maybe the 2K resistor is a good Open Source Hardware Software trade-off: the maximum error due to leakage of pA is 0. The test The PCB made for the op-amp test from the previous post was used for the test. The limiting resistor was 5K 2x10K in parallel. For the test with the current sink, I added an external PNP transistor and a diode. The pictures show when the current sink circuit is used, the current which flows through the diode and goes to the common ground.

This is visible in the current measurement of the DPS When the current sink circuit is not used, the current is "missing" on the DPS display, but the KEW multimeter shows that it actually exists. The test results show, that this extra current is the reason for increasing positive power rail. The red solid line is voltage on the positive power rail with current sink circuit.

The voltage was stable at 2V during the whole ramp-up. However, if we look into the blue solid line without current sink , we can see during ramp-up of the input voltage went from 21 to 32V, the positive power supply was increased from 2V to 2. The dependency is very linear and will be further increased if we continue to rump-up.

The same linear increasing of the voltage is observed for the voltage clamp when the current sink circuits is not used the blue dash line. However, the red dash line which is the voltage clamping value with current sink circuit shows stable value across V range.

Using linear regression, I can estimate increasing the voltage clamp up to 4V for 40V input voltage and up to 6.

This can destroy the op-amp if there is not enough head-room of the power supply. There is no difference in the current passes through the resistor and the diode. This is visible with the blue and red dot lines. Here the negative power rail voltage is constant during the input voltage ramp-up. The clamp voltage is also relative constant after input voltage exceeds the power rail and diode's forward voltage.

Labels: Schematics , Voltmeter front end. In this post I want to find out if a single buffer op-amp can replace the classic schematics used in the old days of DMM. I tested some parameters which are critical for the analog front-end. Some of them are given only as typical values and I want to know what is maximum value I can expect.

This is because back then a single op-amp that can satisfy all criteria for error budget, was not available. Mainly these errors are related to input bias current and noise.

This produces other problems which were resolved with additional circuits. More information can be found in the book "The Art of Electronics". I want to avoid this complexity, so I can find if newer op-amps are good enough. Selection of op-amps. In the previous post I made error budget for op-amp parameters and now I have to select available op-amps on the market. I made 3 groups of them :. In the tables below, the parameters are sorted by my priority given in the first column.

All values which do not fit in my error budgets are colored in red. The error budget is: Ibias less then 50pA Voltage noise , p2p value less than 4. This is equivalent to 7 digits.

The test for this parameter will be performed with short between positive input and the ground. My criteria for this threshold is to reach a 6 digits when the input resistance is 1MOmhs and take into account the previous voltage noise thresholds. Combining voltage noise, current noise density and the input resistance must provide at least 6 digits for 12V or 22V ranges when input resistance is 1MOhms or at least 7 digits when the inputs are short.

In the case of the 10V range, 12V over-range, the value is 24V and in the case of the 20V range, 22V over-range, the value is 44V. Details in the datasheet should be read carefully, because they are not always obvious. For example :. I bought 2 Open Hardware Voltmeter Windows pcs. Later I realized that it is not the perfect PCB and the second version is already developed, but I have to wait for the end of the current world pandemic situation. In the first PCB version I forgot :.

Here is the layout of the second PCB version:. Vcm using Source Meter Unit. This characterization test can be seen in many datasheets. Unfortunately I do not have such equipment, therefore I can't make a test. I looked for low-cost solution and I found an evaluation board of AD parametric measurement unit, but the lowest range was 5uV that required external 24 bit ADC to make pA measurements. I made more than 20 measurements with 50 Open Hardware Summit 2019 Data samples each for the R and HP and 2 measurements with 50 Open Hardware Plc 360 samples for the rest of the DMM.

You can see from the data, that RT and Keithley lost one digit when 1MOhms resistor was attached to the op-amp's input. The next table shows the tests for the following parameters: Vos, Ibias, Linearity, Voltage RMS and peak to peak noise when inputs are shortened and when 1MOhms input resistor is used.

The first few bias current measurements of the MCP6V51 where far away from the specification limits 5. Results with test board ver. Probably the reason is the missing sense connection from the op-amp output to the resistor and the voltage drop over the resistor leads can worse the linearity measurements.

I already made correction in the test PCB ver. Based on my initial tests, I can answer to my question in the beginning of this post: single buffer op-amp solution as front-end is feasible, but test of the buffer op-amp is required. Relaying only to typical or even maximum parameter's values in the datasheet is not enough. This post is still in progress. Once I get the test PCB ver. Older Posts Home. Subscribe to: Posts Atom. V ref, V. Peak to Peak. V supply, V. Max reverse voltage, V.

Max protection, V. Max current, mA. Limit resistor, Ohms. Limit current, mA. Limit Resistor Power, W.

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