Stable -5VDC Offset Design - Page 1
Forum section: electronics / projects

author: aw_rw1
author_rank: Contributor
timestamp: May 05, 2022, 03:53:39 pm
content: I would like a -5VDC offset for an input signal. I am not sure how to do this. What would be the best way to do this? The offset should be very stable.What do you think about using a AD688 +/-10VDC voltage reference with a voltage divider? Or using a AD688 with a zener at the -10V?

author: TimFox
author_rank: Super Contributor
timestamp: May 05, 2022, 03:59:38 pm
content: How stable do you require?A decent voltage reference (of which there are many), or a TL431 shunt regulator, will be far more stable than a Zener diode.Are you using an operational amplifier at the input? Do you have a split supply available? What bandwidth is required?

author: aw_rw1
author_rank: Contributor
timestamp: May 05, 2022, 04:21:17 pm
content: TimFox -QuoteThe input signal is coming from a 4-20mA current going through a 500 ohm resistor. I am not using an opamp for this input, but I would use a voltage follower after the input is shifted.QuoteI have a split supply for this project. It is a 490-PDM1-S24-D9-S. It produces +/-9VDC. Although, I seen a +/- 0.035VDC variance on the DC/DC after power up. From the DC/DC's datasheet, its output accuracy could vary between -7.5% to +10% depending on load.QuoteI don't have a bandwidth requirement. The signal is quite slow.

author: TimFox
author_rank: Super Contributor
timestamp: May 05, 2022, 04:37:55 pm
content: Please clarify your voltage shift.4 to 20 mA through 0.5gives 2 to 10 mV (rather small compared with your 5 V statement).What output voltages do you want the 2 to 10 mV range mapped into, and do you have a gain in your "voltage follower"?Again, you say "stable". What range of instability can your application tolerate?There are lots of ways to get a reasonable stable voltage from your 35 mV variation in the +/- 9V supply. Unfortunately, available three-terminal voltage references are all positive (to my knowledge from a few years ago), so a two-terminal device such as the TL431 or its more expensive brethren may be needed.

author: magic
author_rank: Super Contributor
timestamp: May 05, 2022, 04:47:52 pm
content: Quote from: aw_rw1 on May 05, 2022, 03:53:39 pmI think that with a ±9V supply there isn't enough voltage for this chip and AD586 could be infinitely more useful in such application, or at least AD588 if you really have a genuine need to use something exotic and expensiveQuote from: TimFox on May 05, 2022, 03:59:38 pmNot a bad question to ask.Quote from: TimFox on May 05, 2022, 04:37:55 pmI see 500Ω, so it's 2~10V.Also, observe that 10V is more than 9V, so the level shifting may get tricky having to work with signals outside the supply span.

author: aw_rw1
author_rank: Contributor
timestamp: May 05, 2022, 04:52:43 pm
content: QuoteSorry, the resistance should had been 500 ohms, not 0.5. So the input voltage would be 2-10VDC. (Above response has been modified.)QuoteI am mapping the 2-10VDC to +/-9VDC. So yes there should be a gain to the voltage follower. The gain would consist of a noninverting amplifier configuration.QuoteWe are looking at +/-1mV stability for a +/-10 signal. So accuracy needs to be 0.01%. Is the application asking for too much?QuoteHave you looked at the AD688? It is a 3ppm/c device. Super expensive, but we only need one.

author: TimFox
author_rank: Super Contributor
timestamp: May 05, 2022, 05:00:58 pm
content: Yes, there are better negative references. However, the AD688 is designed to give +10 and -10 V (tracking) outputs from +/- 15 V supply, which won't work with your +/- 9 V supply.However, as pointed out above, shifting the signal to a range of -9 V to +9 V with a +/- 9 V supply is not reasonable.Can you tolerate a smaller output span, such as -7 V to +7 V? Or change to a +/- (12 or 15) V supply?If you have a non-inverting amplifier with appropriate gain, a voltage follower following that amplifier is not needed.

author: aw_rw1
author_rank: Contributor
timestamp: May 05, 2022, 05:02:18 pm
content: Please, see attachment for circuit diagram. This should give a better idea than text.

author: TimFox
author_rank: Super Contributor
timestamp: May 05, 2022, 05:04:26 pm
content: Better to connect a positive reference voltage (buffer optional) to the "bottom" of R1 in the second amplifier.RA and the other buffer are also not needed.The calculations of R1 and R2, and the reference voltage, are left as an exercise for the reader, but you won't get +/- 10 V output with a +/- 9 V supply.

author: aw_rw1
author_rank: Contributor
timestamp: May 05, 2022, 05:18:23 pm
content: QuoteHow would I go about doing this? AD588?QuoteI will look into other supplies. The max/min vo is +/-11VDC. So I could use +/-10VDC supply to be safe.

author: TimFox
author_rank: Super Contributor
timestamp: May 05, 2022, 05:38:12 pm
content: Check that the output of your chosen amplifier can swing to your desired output voltage with your power supply.There are many positive three-terminal reference ICs designed to put out a very stable voltage into light loads (e.g., < 1 mA) at discrete design voltages from, say, 1.25 to 10 V.After choosing physically-realizable amplifiers and power supply, choose the two resistors R1 and R2 to get the appropriate gain from your burden resistor to the output, then calculate the voltage at the bottom of R1 needed to get the two extreme values for the two extreme inputs. If that voltage is too low, you can replace R1 by a voltage divider from the reference chip.

author: aw_rw1
author_rank: Contributor
timestamp: May 05, 2022, 05:45:29 pm
content: QuoteI got it. Thanks for the help.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 06, 2022, 10:06:08 pm
content: I don't know how stable you want it, but one option is to use a 5V voltage reference in shunt form (such as TL431 with two resistors, but TL431 is not extremely stable) and then tie the other end via a resistor to some negative voltage.

author: Terry Bites
author_rank: Super Contributor
timestamp: May 07, 2022, 01:40:20 pm
content: This kind thing .

author: aw_rw1
author_rank: Contributor
timestamp: May 17, 2022, 03:57:54 pm
content: QuoteThe ADR4550 looks good too.