Below-unity Op-Amp attenuator vs. voltage divider  - Page 1
Forum section: electronics / projects

author: jt
author_rank: Regular Contributor
timestamp: February 16, 2016, 04:23:52 am
content: I was hoping for some enlightenment as to what advantages/disadvantages are involved in different approaches to scaling-down a low frequency single-ended analog signal.I'm specifically interested in what advantages a below-unity inverting op amp configuration (Rf < Rg) might have vs. a resistor divider output buffered by a voltage-follower. The part count is the same, the input/output impedance is similar, the resistor tolerance/drift implications are similar and according to the following, there doesn't seem to be much difference in stability concerns.https://e2e.ti.com/blogs_/archives/b/thesignal/archive/2013/02/04/the-inverting-attenuator-g-0-1-uh-oh-is-it-unstableFor the sake of discussion, lets say that I don't care about the sign inversion of the inverting op-amp configuration.Is there different Johnson noise implications? Does one configuration have greater bandwidth performance over the other? Am I missing anything?----Edit: Since the discussion is starting to get into the details, I thought I'd post some application specifics, although I am still curious as to what the trade-offs might be (even if they are not applicable to this specific use case).I have a sensor input that varies from 0-5VDC with <10Hz bandwidth. I need to apply this signal to an ADC driver amp that needs 200mV headroom from the local 0 and 5V rails. So I need an attenuate & shift operation. I'm really not power/cost/board space constrained. I'd like to maximize precision, but I don't have any hard requirements.So for this application, it probably makes little difference which topology I choose, but I am curious what the trade offs are if there are any. So far it sounds like there isn't anything major and the optimal solution is dependent on the specifics of an application.

author: sarepairman2
author_rank: Frequent Contributor
timestamp: February 16, 2016, 06:22:11 am
content: if your system is input impedance sensitive but not so much gain sensitive and you don't like adjustable resistors a target input impedance is easier to meet with a inverting amplifier then a divider, since Ri of Inverter = input impedance, while the divider is R1 + R2i.e. you do digital gain correction with auto software cal but you want a STANDARD 50 ohm input impedance, you can use a precision 50 ohm Ri on a inverter, set the gain to whatever with the closest available part and digitally correct.If you want to do that with a divider you are stuck with using thermally matched parts, for input impedance stability purposes.thermally matched parts are not really available in whatever ratio you like, so you are stuck with few options if you want a total input impedance of X and a attenuation of Y. Easy to imagine that even with digital gain correction you will run out of scale.Trimpots are not available in 0.01% accuracy (i'm not even talking drift !). so you will be stuck using like say 1% matching (if that) on your input impedances between units if you go this route.if your system is also gain drift sensitive then you still need thermally matched parts for the inverting amplifier, so I don't know.frustrating to think about this comparison, something tells me it should be totally obvious but its not

author: jt
author_rank: Regular Contributor
timestamp: February 16, 2016, 07:15:18 am
content: Thanks sarepairman2.I agree it is a frustrating comparison - I really feel like the inverting amp should be better, but I'm having trouble justifying it. For my application, a precision input impedance isn't needed.I did notice there are indeed limited ratio options for thermally matched resistors. I'd like to get a gain of about 0.8, and so far it looks like I'd need to do this in two stages: divide by 5 and multiply by 4.Maybe one minor difference is the affect of the input offset voltage of the op amp. With the resistor divider, the offset is added to the output by the voltage follower. For the inverting amp, it will get attenuated by your Rf/Rg ratio (I think). This difference seems fairly minor though, at least for my application.I wonder if there is a clear advantage between the two topologies if the input is differential instead of single-ended? Nothing imediately comes to mind...

author: sarepairman2
author_rank: Frequent Contributor
timestamp: February 16, 2016, 07:31:18 am
content: i wonder if the attenuator noise gain figure is dependent on op amp toplogy

author: Vgkid
author_rank: Super Contributor
timestamp: February 16, 2016, 07:57:46 am
content: Are you working with dc or ac, and what voltages.

author: gnasirator
author_rank: Contributor
timestamp: February 16, 2016, 08:13:13 am
content: One advantage of a voltage divier that I can think of would be that it can handle far greater input voltage ranges and scales them down passively into usable values (e.g. <5V).There are OpAmps with high acceptable CMRR ratings (AD628) but they're expensive and still limited.So if you want to measure mains voltage, I'd go resistor divider.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: February 16, 2016, 08:18:29 am
content: JT,Take a look at this solution:I am using 1/2 of a LT5400-7 resistor array. The datasheet can be found here:http://cds.linear.com/docs/en/datasheet/5400fc.pdfThe resistors are in ratio 4:1 to get a division ratio of 0.8This may or may not work. You have to consider loading on the input and output and power dissipation in the resistors.Regards,Jay_Diddy_B

author: Kalvin
author_rank: Super Contributor
timestamp: February 16, 2016, 08:46:33 am
content: If the signal level is suitable for the op amp, you can use the op amp as a high impedance unity gain buffer and feed the signal into the op amp. Then, place the resistor divider to the op amp output. This construction will give you high input impedance and you can select convenient resistor values as the resistors will not load the input. Optionally, use another op amp as a high impedance buffer to the resistor divider output if needed.

author: rob77
author_rank: Super Contributor
timestamp: February 16, 2016, 09:16:51 am
content: aren't most of the opamps unstable below unity gain ?

author: Fungus
author_rank: Super Contributor
timestamp: February 16, 2016, 09:49:37 am
content: Quote from: Kalvin on February 16, 2016, 08:46:33 amThis.

author: dom0
author_rank: Super Contributor
timestamp: February 16, 2016, 09:57:26 am
content: Quote from: rob77 on February 16, 2016, 09:16:51 amNo.Stability depends on noise, not signal gain. The non-inverting amplifier has an obvious lower signal gain bound of 1 (and noise gain >= signal gain, necessarily [and provably]), similarly the inverting amplifier has a lower *noise* gain bound of 1 (not as obvious as the non-inverting case, from an analysis point of view, but intuitively pretty clear ; how would we reduce input errors with passive feedback? It is not possible).So if you use an inverting amplifier with a gain of 0.8, the noise gain is actually 1.8.

author: jt
author_rank: Regular Contributor
timestamp: February 16, 2016, 02:52:54 pm
content: Quote0-5V <10Hz BWQuoteThis is a good read. According to the comments section, there is a bit of misinformation being perpetuated.https://e2e.ti.com/blogs_/archives/b/thesignal/archive/2013/02/04/the-inverting-attenuator-g-0-1-uh-oh-is-it-unstableQuoteAh, that is almost perfect. I need an input impedance of >10k-ohm. Perhaps I should just use an input buffer as Kalvin and Fungus have suggested. I did find a Vishay option that is on the edge of this requirment that is tempting. I need 10 channels of this signal chain so it would be nice to minimize the part count.http://www.digikey.com/product-detail/en/MPMT20018001AT1/MPMT-2K%2F8KCT-ND/1771082

author: mstoer
author_rank: Contributor
timestamp: February 16, 2016, 05:35:26 pm
content: Quote from: jt on February 16, 2016, 02:52:54 pmThanks for that article, I saved that and next time I am told not to make an o-amp attenuator I'll refer them there. I usually do what Kalvin posted, a combination of buffers and dividers as needed.

author: sarepairman2
author_rank: Frequent Contributor
timestamp: February 17, 2016, 07:39:33 am
content: how about a noise/parasitic behavior analysis? that is still interestingdivided buffer vs buffered divider vs inverter

author: dom0
author_rank: Super Contributor
timestamp: February 17, 2016, 10:13:52 am
content: With similar values of resistance the inverting attenuator has higher noise (/drift/offset). See my post above.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 18, 2016, 05:03:51 pm
content: Usually the divider has the better performance (noise, offset) compared to the inverting amplifier. The inverting amplifier can have advantages when is comes to switching the amplification by changing the feedback to different resistors.The inverting amplifier also makes overvoltage protection very easy, as the can just shunt to ground and with normally zero voltage, the diodes don't have much leakage.