One of the most common analog signals used in process control applications is the 4-20 mA signal. There are times where a 4-20 mA has to be generated and transmitted by a controller. Here is picture of an 8 channel 4-20mA output interface, the VP-8AI.

This interface is powered by a PIC18F45K20 micro, has RS485 Modbus RTU protocol, and the output dual DA Converters are the MCP4922 DAC which has 12 bit resolution and an SPI interface.

Here is a simplified schematic of one of the outputs from this interface:

Here is how the math works:

1. Typically, the DAC will run from 3.3VDC and the reference will also be 3.3VDC

2. To allow for overange and "headroom", the full scale Vin will be 3 VDC which corresponds to 37214 DA Counts (3/3.3 * 4096)

3. Basic Op Amp theory states 3 VDC Vin will then appear across R1 (let's say the standard 499R resistor is actually 500 Ohms to make everything cleaner)

4. Current flow through R1 will be 3VDC/500 Ohms = 0.006 A or 6 mA.

5, The same 6 mA will flow through R2, giving a voltage across R2 of .006A * 330 Ohm = 1.98 V

6. Basic Op Amp theory states that the voltage across R2 will equal the same voltage across R3, therefore the current flowing through R3 = 1.98 V / 100 R = 19.8 mA for Aout

With a little bit of tweaking on the Max defined DA Counts for 20 mA, the above circuit works exceptionally well. We use this in various products in our industrial line and also in the Widgetlords product offering as follows:

The V+ power for this type of circuit is typically 24 VDC to allow for output "Drive". Typically process instrumentation that accepts 4-20mA signal will have "Load" resistor of 250 Ohms (or less). At 24 VDC power and given that the above circuit will have a voltage loss of 2 VDC (give or take), the above wold be able to "Drive" up to 4 "Loads" in series. (Each Load resistor at 250 Ohms with 20 mA flow will have 5 VDC across the load). Simple, and clean.

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