Electronics > Projects, Designs, and Technical Stuff
ICL7135 ground loop
davorin:
Good afternoon (o;
Almost finished a thermocouple design for reading out temperatures via GPIB and/or USB...
Now as I want to build linecards for a flexible measurement system a voltmeter is also a must..so that's where the ICL7135 comes in (o;
Let's say the ICL7135 is read out by a MCU which then sends data down to the host PC either through GPIB or USB....
Now when I measure a voltage from a device which is a USB device and therefore the ground is connected to the host PC as well as the measuring device...
datahseet mentions something about the floating inputs must go above +v - 0.5V and below -V + 1V....
Do I get false reading from the two floating analog inputs of the ICL7135?
Or better doing an isolated design like those good old HP 3456A voltmeters? (o;
Kleinstein:
When doing measurement and send to the PC, it is a good idea to have some isolation. The usually easiest way is to have UART output from the µC , than some isolated coupler (opto or ADMU....) and than an UART to USB bridge chip. The USB can power the UART to USB chip easy. For the rest of the circuit it may need an extra DCDC converter or an extra supply / wall wart.
The inputs of the ICL7135 are differential, but the common mode range is limited. It may work without isolation, but it can fail if there is a lot of common mode voltage, especially with a Laptop or similar that is not directly connected to ground.
davorin:
Ah okay...well I already looked up some isolated DC/DC converters from Digikey....not very expensive...
though not sure if the introduced ripple messes up with the results...but then again...the intended system is more for measuring several
DC voltages at once in the range of 100V - 400 (/me repairing old vacuum tube radio ;o).
For connecting the ICL7135 to the host interface I'll use a cheapo MCU with CAN and isolated CAN transceiver..probably NXP....
that way I can have several voltmeter cards in the same housing....or other cards doing temperature measurements....
I assume some protection diodes on the inputs might be needed as well to protect from too much floating away...
Well first I am curious how the USB K-thermocouple PCBs turned out which I should get on Monday from aisler.net....
mikerj:
--- Quote from: davorin on June 19, 2020, 01:58:35 pm ---For connecting the ICL7135 to the host interface I'll use a cheapo MCU with CAN and isolated CAN transceiver..probably NXP....
that way I can have several voltmeter cards in the same housing....or other cards doing temperature measurements....
--- End quote ---
The ICL7135 has mutliplexed BCD outputs which are rather annoying to interface to a micro, why wouldn't you choose an ADC with an SPI or I2C interface?
Kleinstein:
The ICL7135 is nice for display oriented use, but a little awkward when using with a µC. With an µC I would consider a more binary ADC like MCP3421 (has a reference inside, I2C interface, internal gain, smaller and no bulky caps needed, however not so high in impedance).
With an DCDC converter the ripple is usually not the really bad part. This can be filtered relatively easy: DCDC -> filter -> LDO. The nasty part is the common mode injected signal. Here filtering with a common mode choke is only partially effective, as the ideally is no low impedance path to ground.
The input would need a divider (for the high voltages) and some diodes for clamping. The lower voltage ranges may want an extra (possibly zero drift) amplifier, especially with the 7135.
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