Initially I thought to have 5A (1mA resolution) and 1mA (1uA resolution) ranges with 0.02 mV/mA burden voltage. This is fine for 3.5 digits DMM, but after few months of research and results from testing with the breadboard, I decided to revisit the resolution with better ones. In result, the burden voltage will be increased, but will be not greater then the DMM on the market.
To keep things simple, I decided to make research only for 2 current ranges with 2 separate binding posts. I was not able to find suitable relay with low contact resistance, low leakage and high current which to allow usage of a single binding posts for both ranges. The first range will be from 1uA to 2A (mA/A range) with 0.1 Ohms shunt and the second will be from 1nA to 2 mA (nA/uA range) with 100 Ohms shunt. For the first DMM PCB prototype only the mA/A range will be implemented.
The 0.01 Ohms shunt is ideal for 2A range due low burden voltage, but I choose 0.1 Ohms resistor due the following reasons:
- I was not able to find a very fast acting fuse, which have specified resistance for nominal current less than 0.1 Ohms. If the shunt is less than 0.1 Ohms, the voltage drop will be determined by the fuse instead of the shunt. Most of the fuses, specified the cold resistance when 10% of the nominal current is used.
- This the the minimum value, which can give 1uA resolution with gain 10 amplifier for 6.5 digits voltmeter. If the shunt is 0.01 Ohms, gain of 100 is required to have 1uA resolution, but in this case ultra low noise op amp are required, which will cause bigger bias current and/or input voltage offset.
I found the following available 0.1 Ohms shunt resistors in the market:
To keep things simple, I decided to make research only for 2 current ranges with 2 separate binding posts. I was not able to find suitable relay with low contact resistance, low leakage and high current which to allow usage of a single binding posts for both ranges. The first range will be from 1uA to 2A (mA/A range) with 0.1 Ohms shunt and the second will be from 1nA to 2 mA (nA/uA range) with 100 Ohms shunt. For the first DMM PCB prototype only the mA/A range will be implemented.
The 0.01 Ohms shunt is ideal for 2A range due low burden voltage, but I choose 0.1 Ohms resistor due the following reasons:
- I was not able to find a very fast acting fuse, which have specified resistance for nominal current less than 0.1 Ohms. If the shunt is less than 0.1 Ohms, the voltage drop will be determined by the fuse instead of the shunt. Most of the fuses, specified the cold resistance when 10% of the nominal current is used.
- This the the minimum value, which can give 1uA resolution with gain 10 amplifier for 6.5 digits voltmeter. If the shunt is 0.01 Ohms, gain of 100 is required to have 1uA resolution, but in this case ultra low noise op amp are required, which will cause bigger bias current and/or input voltage offset.
I found the following available 0.1 Ohms shunt resistors in the market:
All of them are made by Vishay Precision Group and they are available in single quantity from Mouser, Digi-key and Avnet.
Note that sub-ohm shunt resistor must have 4 wire connection to reduce the measurement error with elimination of the lead and contact resistance.
The specified maximum load life (accuracy) is for 2000 hours uninterruptible working at power rating and 25°C or 70°C depends on the model. When the DMM is not used continuous or the measurement current is less than the maximum, these values are several times less. I was not able to find such data for Vishay current sense resistors, but in the datasheet of the PCR series from Riedon is shown that the accuracy can be increased 5 times (from 0.5% to 0.1%) if the current sense is used at 60% from the rated nominal power. So may be it make sense to select a current sense resistor with the maximum power rating and to use it in lower current then the nominal. Also using heat-sink can lower the the resistor's temperature, thus the load life can be improved.
I think that the best price-quality ratio is the VCS1625P series. It will have sub 500 ppm accuracy for one year (5 days/8 hours working condition) when it is used in 10°C temperature window. The maximum current is 5A, which can allow to use high rating fuse with less resistance. The maximum power for the 2A range will be 2*Imax*Rshunt = 2*2*0.1 = 0.4W which is in the range.
The next thing is to select the fuse for mA/A range. I looked for very fast acting, low resistance fuse in the A range. Most of datasheets have values only for the cold resistance (10% of the rated current). Only for EATON Bussmann series I found fuse resistance values when nominal current is applied, which they called: "typical voltage drop measured at 25°C±3°C ambient temperature at rated current". For the 5A maximum current of the VCS1625P shunt, according the Time-Current curve chart, using GBB-2-R model (2A rate current), the fuse will break in 50ms when 5A current is applied. By this way it is guarantee that the shunt will be not overloaded with more than the nominal current value and the shunt accuracy will not exceed the datasheet value. The cold resistance of the GBB-2-R is 0.0662 Ohms, the resistance for nominal current is 0.1687 Ohms. Combined with shunt resistor, the input DMM resistance for current measurement will be 0.16632 Ohms for <0.2A and 0.2687 Ohms for >0.2A and <2A. This does not include the connecting wires, PCB trace and binding post resistances. Probably the final burden voltage will be 0.2 mV/mA for <0.2A and 0.3 mV/mA for > 0.2 A. One alternative is the Littlefuse 459 Pico series 2A model. It will break in 20ms when 5A current is applied. Unfortunately datasheet only specified the cold resistance which is 0.0468 Ohms, which is less than the Bussmann series.
Both EATON (DMM-B) and Littlefuse (FLU) have fuse series for DMM usage, but in the specification files, there is no information about the fuse resistance.
For the uA/nA range I choose 100 Ohms shunt resistor. It will generate 0.1uV voltage drop for the 1nA current, which again should be multiplied by 10 to get 1uV minimum resolution. For this range, the fuse resistance is not so much important, due high value of the shunt resistor.
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