Strange output of CMOS Voltage Converters - Page 1
Forum section: electronics / beginners

author: Tommy1984
author_rank: Regular Contributor
timestamp: February 06, 2018, 09:49:44 pm
content: Hi everyone!I made the circuit as shown in the picture below. My problem is, even if the input voltage is +5V, i get -4.9V on the output.I followed all the tips and suggestions in the datasheet, but I guess I still missing something!Any suggestions? THANKSTommy

author: grifftech
author_rank: Frequent Contributor
timestamp: February 06, 2018, 10:00:12 pm
content: The output voltage is within 2% of what it should be, it is fine

author: Tommy1984
author_rank: Regular Contributor
timestamp: February 06, 2018, 10:02:22 pm
content: Quote from: grifftech on February 06, 2018, 10:00:12 pmThank you for your reply!Can't I get exactly as the input? Because I need it to power an op amp, and I guess it's better if it's closer to the positive oneRegards,Tommy

author: ogden
author_rank: Super Contributor
timestamp: February 06, 2018, 11:49:18 pm
content: Quote from: Tommy1984 on February 06, 2018, 10:02:22 pmIt will be fine for most of the applications. As long as output swing stays within desired specs, your positive/negative supply voltage precision does not matter so much.

author: Buriedcode
author_rank: Super Contributor
timestamp: February 07, 2018, 12:50:46 am
content: The bipolar supply for opamps is to allow the output (and input) to swing above and below 0V. This makes many applications easier as AC signals don't have to have DC bias. As long as the input/output doesn't need to go lower than the negative voltage minus the output (opamps can only go within a certain voltage of either rail, for some this is ~1.5V, for rail-to-rail opamps this can be a few tens of mv) it will be fine.Also note these charge-pumps can only output a voltage higher than the negative of the power supply. They also have fairly high output resistance - so the more current you draw from the negative rail, the higher (less negative) the voltage will go.What opamp are you using? And what is the application?

author: Tommy1984
author_rank: Regular Contributor
timestamp: February 07, 2018, 09:03:48 am
content: Quote from: Buriedcode on February 07, 2018, 12:50:46 amThank you for your reply!Actually I’m using OP277 to amplify a DC signal with gain of 1000.Because the signal is so low, about 50 uV, I thought it will be better if I power the op amp with dual supply.What’s the maximum difference between the dual supply which is allowed so it doesn’t effect on the output, gain error... etc?Regards,Tommy

author: daqq
author_rank: Super Contributor
timestamp: February 07, 2018, 09:30:10 am
content: QuoteThe ICL7660 (and similar) can create some heavy noise on the data lines - take a look at your -5V voltage with an oscilloscope. Be sure that the op amp power supply rejection at the operating frequency is good enough for your application. You may need to employ some heavy filtering.Are you sure that you can't just shift the reference signal? Amplify said 50uV but rather than doing it against GND you do it against, say, 2.5V (VCC/2)?

author: Tommy1984
author_rank: Regular Contributor
timestamp: February 07, 2018, 10:13:12 am
content: Quote from: daqq on February 07, 2018, 09:30:10 amThank you for your reply!What did you mean by shifting the reference signal? By powering the op amp by+/- 2.5V?Regards,Tommy

author: daqq
author_rank: Super Contributor
timestamp: February 07, 2018, 10:37:32 am
content: QuoteNope, in a classic simple non inverting amplifier you tie the feedback divider to ground and measure your signal against ground. Ground is also where you power your negative op amp rail.Instead of adding another power supply, you can tie your feedback to a different potential, say, +2.5V and measure the output against this voltage. You of course have to tie the other end of the input source to said +2.5V voltage as well. Basically you shift your signal "zero/reference potential" this way.

author: Tommy1984
author_rank: Regular Contributor
timestamp: February 07, 2018, 10:40:38 am
content: Quote from: daqq on February 07, 2018, 10:37:32 amGreat suggestionI'll try it when I'm back home and give you a feedback!Regards,Tommy

author: David Hess
author_rank: Super Contributor
timestamp: February 07, 2018, 05:18:56 pm
content: Quote from: Buriedcode on February 07, 2018, 12:50:46 amThis completely explains the slightly lower (toward ground) negative output voltage and it is rarely an issue when powering an operational amplifier.Quote from: daqq on February 07, 2018, 09:30:10 amCMOS charge pumps are often quieter than a switching regulator doing the same job. Route the current return paths correctly and there should not be a problem with noise. If extra filtering is needed, it is not difficult to do.