Which Op-Amp for a VCCS ?  - Page 1
Forum section: electronics / beginners

author: doublec4
author_rank: Regular Contributor
timestamp: May 31, 2019, 03:19:17 pm
content: Hi All,I have a few sensors here that output 0-5V analog. I have a SCADA modem that has analog inputs but requires 0-20mA input.I have been searching how to properly do this and came across this here:https://www.allaboutcircuits.com/textbook/semiconductors/chpt-8/voltage-to-current-signal-conversion/Explanation is great and makes sense to me, but since I've never played with an op-amp I'm a little lost as to what characteristics I'm looking for to perform this function in this "transconductance amplifier" circuit. I'm having some trouble finding info to guide me through this process.Any help would be appreciated!Thank you

author: ogden
author_rank: Super Contributor
timestamp: May 31, 2019, 03:36:59 pm
content: You may start with typical 4-20mA circuit,like this one(search result)

author: doublec4
author_rank: Regular Contributor
timestamp: May 31, 2019, 06:01:51 pm
content: Thanks ogden,I checked out the datasheet and the part on Digikey.I guess what I'm looking for is more of a "why" this part number? This circuit is for 0-3V input, but if I want to adapt for 0-5V reading I'm not sure what I'm looking for in the parameters on Digikey to do so, or what else to modify in the circuit based on my conditions.I will have a 24VDC supply for the SCADA modem and a 5VDC supply available for the sensors.Also, the complexity of this circuit is greater than the original one I posted. Is this for greater precision? I should mention that my application is not for precision instrumentation, so I'm not sure if that changes anything. I am fine with some reasonable amount of error.I will keep searching for an explanation in the meantime, but if anyone knows of a good link to understand how to go about choosing the components to do something like this it would really help further my understanding.Thanks!

author: schmitt trigger
author_rank: Super Contributor
timestamp: May 31, 2019, 06:14:42 pm
content: Go to the Analog devices website.They have a whole table of 4 - 20 mA loop interface ICs. There are app notes that explain the underlying concepts.

author: ogden
author_rank: Super Contributor
timestamp: May 31, 2019, 08:54:13 pm
content: Quote from: doublec4 on May 31, 2019, 06:01:51 pmDunno. As I said - it did pop-up in the search. If it were another part number - would you wonder less?That seems to be old circuit/appnote referring to old opamp. Most likely you can use any generic opamp.QuoteFor 0-5V input you need to 1) increase supply voltage from 5V to ~7V and 2) change voltage divider.

author: doublec4
author_rank: Regular Contributor
timestamp: June 12, 2019, 04:33:30 pm
content: I've had minimal time lately but I really do intend on continuing to read up on how this works. This OP amp basics video has been helpful so far:https://youtu.be/7FYHt5XviKcHowever, being strapped for time, I need something "that just works" while I continue to learn how to build my own.I found this on amazon:https://www.amazon.ca/MagiDeal-Voltage-Isolation-Transmitter-Converter/dp/B07F1P576L/ref=asc_df_B07F1P576L/?tag=googleshopc0c-20&linkCode=df0&hvadid=335365451913&hvpos=1o5&hvnetw=g&hvrand=16048267424708276403&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9000935&hvtargid=pla-750133362629&psc=1Seems to fit the bill, 0-5V in, 24V supply, with 4-20mA out... but it is only a single Iout ... the device I am monitoring with requires a current "loop" .. two wires... maybe a stupid question but what am I missing here? It also has adjustment pots and no datasheet so that is helpfulAlternatively, I also found this:https://store.ncd.io/product/ad5696-4-channel-4-20ma-16-bit-current-loop-transmitter-with-iot-interface/Seems like I can monitor my sensors with Arduino and use I2C with this board to output a 4-20mA signal. In the "drawings" tab of this product, it shows how the external power (ex:24VDC) would be put in the loop with the device being controlled. Maybe this is a better approach?EDIT:See pg 27/28 for the device I am trying to connect to. You can see where it shows the analog input and the description talks about a current loop with externally supplied power:http://www.tetramodem.com/fileadmin/documents/Dokcumentations/TMO-100_E.pdf