Op amp selection help. - Page 1
Forum section: electronics / beginners

author: BradN
author_rank: Newbie
timestamp: September 16, 2018, 03:18:55 pm
content: I am looking for a rail to rail opamp with a 10v or 12v single supply in a dip package. I need this to amplify the signal from a MCP4727 DAC to 10V to control a piece of machinery that requires 0-10v control signal. I have little experience with analog electronics and have been wading through rivers of opamp data sheets and am still not sure if I am selecting the correct devices. Any suggestions would be appreciated.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 16, 2018, 03:55:28 pm
content: A common RR op for up to 16 V is the TS912, though only dual version available.If the supply is 12 V or higher, one might get away with a single supply OP, maybe even an LM358 (with a light load).

author: Simon123
author_rank: Regular Contributor
timestamp: September 16, 2018, 04:46:52 pm
content: Or maybe just use Voltage inverter to get the negative rail, ICL7650 for example and then use OP07 or TL071 or something ...

author: rstofer
author_rank: Super Contributor
timestamp: September 16, 2018, 05:18:29 pm
content: One of the first things you need to know is the input impedance of the load. Sure, you have to send 0-10V but at what current? It could be quite high and of no concern. But it might not be...Then, since bandwidth is not likely to be an issue, any op amp can do the job.Rail-to-rail is often a marketing term. Given a supply of 0V-15V I'm not sure that you can actually get down to 0V on the output for 0V on the input. There simply must be some voltage drop across the output driver. Given that the supply voltage is higher than the required output (15V supply, 10V signal) there should be no problem on the top end.I would avoid working at the edges. If necessary, I might use -2V +12V for the supply voltages to leave a bit of room at the rails. I wouldn't mind throwing out the negative side and using the far more common -15V..+15V supplies. Or even -12V..+12V.

author: Zero999
author_rank: Super Contributor
timestamp: September 17, 2018, 10:02:09 am
content: Quote from: Kleinstein on September 16, 2018, 03:55:28 pmIt also depends on whether the load sinks or sources current. The LM358's output can't get very near zero, if the output has to sink more than 10µA. If the load is just a 1k resistor to 0V, then it should be fine, as it will only need to source current and 10V out at 10mA, with 12V in is no problem for the LM358. If only one op-amp is required, it's also possible to connect the two op-amps in the LM358 in parallel, to increase the output drive.https://www.intersil.com/content/dam/Intersil/documents/an11/an1111.pdf

author: exe
author_rank: Unknown
timestamp: September 17, 2018, 11:11:02 am
content: Quote from: rstofer on September 16, 2018, 05:18:29 pmWell, that's true. Output swing also depends on the output current. Also, RtR can mean RtR input, or RtR output, or both. Probably, no need for full RtR input here, but common mode should include V-.Sadly, I don't have any part numbers at hand, but there are devices from, e.g., AD, that designed for near-negative-rail output.Also, a rail splitter (e.g. TLE2426) can be an option. I think I saw circuit where an unused opamp was used for it, but can't find the schematic.UP: also a charge pump can be a solution. Like lm7705 or I even saw just a two-lead device that is to be inserted into opamp's negative rail. But I forgot the part number. Although, these are more exotic solutions, availability and cost of rare parts often a no-go. But for one-off project why not?

author: capt bullshot
author_rank: Super Contributor
timestamp: September 17, 2018, 01:10:51 pm
content: For the described application, often a LM358 will do the job. Just provide a pull-down resistor for its output and enough headroom for the positive supply voltage.

author: iMo
author_rank: Super Contributor
timestamp: September 17, 2018, 02:48:39 pm
content: This may work, it needs +15V power, works from around 50mV input from the DAC up, DAC 0.05 .. 5V output, LM358 output 100mV .. 10V, rise time 2us.Machinery "regulation input" estimated 1k input resistance.Simulation only, not tested in real HW.Use at your own riskPS: I cannot find "MCP4727"R1=R2=1k..10k

author: exe
author_rank: Unknown
timestamp: September 17, 2018, 03:36:32 pm
content: What's the purpose of R3 and R4?I guess, R3 slows down the output to avoid any oscillation when driving load over long wires.R4... Is it for stability? Or it lowers the minimal output voltage?

author: David Hess
author_rank: Super Contributor
timestamp: September 18, 2018, 03:38:06 am
content: R3 prevents oscillation since the load is poorly defined and R4 helps pull the output to ground. I might also place a 0.01 microfarad ceramic capacitors across R4 and directly from the output of the operational amplifier to the inverting input.An LT1006 (single), LT1013 (dual), or LT1014 (quad) are a much improved version of the LM358 (dual) and LM324 (quad) which provide active pull-down below 0.6 volts. Any single supply CMOS operational amplifier like an LMC6081 will be suitable also. Very few CMOS operational amplifiers are suitable for use above 15 volts so make sure the supply voltage cannot surge above that.

author: iMo
author_rank: Super Contributor
timestamp: September 18, 2018, 08:58:58 am
content: Do use R1=R2=10k in my schematics above. It saves you 4.5mA output current.Sometimes it is recommended to put R1||R2 into the non-inverting opamp input (ie. 4k7).The output voltage of the LM385 (opamp's output pin) will be 11.15V (because of the R3=100ohm) for 10.0V at "machinery input". Therefore you need 15V Vcc to have enough headroom for the opamp to work properly.QuoteIt slows down the circuit however - ie see below DAC from 1V to 4V and back.