Rail to Rail Op Amp - Page 1
Forum section: electronics / projects

author: phil from seattle
author_rank: Super Contributor
timestamp: December 22, 2020, 09:35:59 pm
content: I'm not an analog guy so this is pushing the limit of my abilities. hoping for your inputs.My application is fairly simple - taking a filtered 5V PWM signal and turning it into a 0-10V Variable Frequency Drive control signal for a spindle. The frequency is basically DC with a small amount of Fpwm ripple. I'm using a simple 2 stage low pass filter with 1.59 kHz cutoff prior to the amp. Then the 0-5V signal is amplified with a gain of 2X to 0-10V. I want to be able to use a 10V supply (VFDs often provide 10V) thus the RtR need. The supply voltage needs to be isolated from the PWM signal circuitry so the VFD supply is a way to avoid an extra power supply. I want to also allow using higher voltage as my customers often have isolated 12 or 24V available. To top it off, I want to stay under 10 mA total consumption.I have learned that the Rail to Rail concept is more of a marketing thing than a reality. The fine print really matters here. I am willing to accept Voh of 200 mV shy of Vcc. That means top spindle speed is 98% of rated max. My customers get cranky when they "loose" speed. My answer has been "use 12V for your supply" but a competitive advantage is not requiring an extra power supply as noted above.I have spent the last couple of days looking at op amps -having trouble finding one that meet my criteria. Looking for:at least 16V supply. Preferably >24VGain Bandwith is not important.rail to rail operation (output swing to within 200 mV of supply)less than $0.35 in parts cost at >= 100 units. (not sure how realistic this is)Chinese disti availability (LCSC).I am finding a fair number of RtR op amps in the price range but it seems like they mostly have a max supply of 6-7 V.

author: Benta
author_rank: Super Contributor
timestamp: December 22, 2020, 10:14:41 pm
content: LMC6482 is my favorite for this kind of thing, but your output swing depends a lot on the load = input resistance of what you want to control. And it's only available as a dual amp.And you won't hit your price point.https://www.ti.com/lit/ds/symlink/lmc6482.pdf

author: Zero999
author_rank: Super Contributor
timestamp: December 22, 2020, 10:36:11 pm
content: Does the input also need to be rail-to-rail. If not, how about the NCS20071?https://www.onsemi.com/pub/Collateral/NCS20071-D.PDFOr the TS912, which has rail-to-rail inputs, as well as outputs.https://www.st.com/resource/en/datasheet/ts912.pdf

author: phil from seattle
author_rank: Super Contributor
timestamp: December 22, 2020, 11:29:27 pm
content: Thanks for the input.In general the input to the VFDs have a fairly low load - <10 mA. The output 10V from them is used to feed a divider potentiometer to supply the 0-10V input speed control. The output 10V is low current (10 mA is a typical spec).The input will be pretty close to 5V as I will have a regulator in the isolated circuit and there is a trimmer pot to control the actual gain of the op amp. The customers will be able to tune it.I can live with a dual opamp - currently using an LM358 for 0-10V (cheap and my current usage doesn't have the RtR requirement).

author: phil from seattle
author_rank: Super Contributor
timestamp: December 22, 2020, 11:43:14 pm
content: Hmm, TS912 looks interesting. LCSC has it for about $0.40 in 100s. JLCPCB (for proto work) only has the TS912B which is $2+. The difference is much better input offset (10mV vs 2 mV). Also, it has the same pinout for the LM358.

author: David Hess
author_rank: Super Contributor
timestamp: December 23, 2020, 04:42:32 am
content: Operational amplifiers built on a 16 volt CMOS process like the mentioned LMC6482 meet your requirements and are pretty cheap if 12 volt operation is acceptable. But beware that when they give an absolute maximum supply voltage specification of 16 volts, they are not kidding; consider including a 14 volt TVS or some other protection.There are many higher voltage operational amplifiers with rail-to-rail outputs but offhand I do not know of any inexpensive ones.With a different circuit configuration, any single operational amplifier can produce a rail-to-rail output with the addition of two bipolar transistors.

author: phil from seattle
author_rank: Super Contributor
timestamp: December 23, 2020, 05:35:26 am
content: QuoteI went looking for this and probably didn't have the right search terms - care to point me in the right direction?

author: Zero999
author_rank: Super Contributor
timestamp: December 23, 2020, 07:55:44 pm
content: Quote from: phil from seattle on December 22, 2020, 11:43:14 pmHow many channels do you need? There's the TS914, which is quad.Quote from: David Hess on December 23, 2020, 04:42:32 amIt sounds like you're speaking from personal experience. It's interesting that they specify the LMC6482 with maximum operating voltage of 15.5V, with an absolute maximum of 16V.The TS912 is around the same price, from most distributors and is specified to work up to 16V, with an absolute maximum of 18V. I've not found anything with a higher voltage rating and both rail-to-rail inputs and an output for the same or less.QuoteThat sounds like asking for instability.

author: phil from seattle
author_rank: Super Contributor
timestamp: December 23, 2020, 08:13:31 pm
content: Quote from: Zero999 on December 23, 2020, 07:55:44 pmActually just need one. Have been using LM358 because it's super cheap. Other than RtR, my application is not at all demanding.

author: David Hess
author_rank: Super Contributor
timestamp: December 24, 2020, 12:08:17 am
content: Quote from: phil from seattle on December 23, 2020, 05:35:26 amThe general idea is shown below. It can be simplified by replacing each current mirror with a transistor and resistor.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: December 27, 2020, 06:02:07 am
content: This is an alternate solution I came up with.Instead of first filtering the signal and then amplifying, I push the PWM through some optocouplers andOn the Front side there are a few LED's in series. Enough that the combined threshold voltage is more then the Vcc, so all LED's are off. Once PWM is added, the LED's (and optocouplers) start blinking alternately, which is transferred to the other side.On the 10V side I've also used 2 resistors. Mainly to prevent the 10V supply from being shorted in any situation.It lacks a bit in linearity, but it's good enough for the inverter I have. ("Yuang hung low" from China).I made a bit of a mistake with the current transfer ratio of the optocouplers, so there is still room for improvement.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: December 27, 2020, 08:45:47 pm
content: What about using a linear optocoupler with a dual photodetector HCNR200-500E? This optocoupler allows one photodetector to adjust the feedback of the OPAMP, and the second to give out a signal. Look at the datasheet - it's a very interesting thing.