Power an op-amp from a DC-DC converter - Page 1
Forum section: electronics / beginners

author: smbaker
author_rank: Regular Contributor
timestamp: September 27, 2015, 07:06:34 pm
content: I have a project where I'd like to power a TL072 with +/- 12V, and only have a single +12V linear supply available. I also have some D121212S-2W DC-DC converters, which I could easily use to generate the -12V op-amp supply from the +12V supply. I know from previous measurement that the output of the DC-DC converter is kinda ugly with some ripple. Let's say I stay away from the rails where the ripple is, and only amplify a +/- 10V signal. My question is, will I get a clean output signal?I plan on breadboarding this up later to find out (can't resist a good experiment), but I'm curious what I should expect to find.

author: Zero999
author_rank: Super Contributor
timestamp: September 27, 2015, 07:10:32 pm
content: It depends on what you're doing.The first question is: do you really need a bipolar supply? Is it possible to configure the circuit for single supply operation?Post a schematic.

author: daqq
author_rank: Super Contributor
timestamp: September 27, 2015, 07:17:37 pm
content: Define clean. Most op amps include parameters such as Supply voltage rejection ratio - this parameter essentially tells you how much crap will be transfered into the output (assuming various parameters). If you are OK with this value, then you should be OK. I don't know however, if that parameter does not increase with less difference between VCC and the output.You can however use a +/-15V DC DC converter and use a low noise voltage regulator to get down to +/-12V with a lot lower noise values.

author: Zero999
author_rank: Super Contributor
timestamp: September 27, 2015, 07:33:25 pm
content: Quote from: daqq on September 27, 2015, 07:17:37 pmThe power supply rejection also depends on the frequency of the ripple and the gain of the amplifier configuration. It will always be better at lower frequencies and gains.You're right, a linear regulator will help but a cheaper option is an emitter follower with a large capacitor between the base and 0V rail. Attached is an example suitable for high currents but it's overkill for what you want so use smaller transistors.

author: SeanB
author_rank: Super Contributor
timestamp: September 27, 2015, 08:00:07 pm
content: Be aware that a lot of opamps have a very bad PSRR on the negative supply side, so make sure that the -12V rail is very low noise. The amplifies capacitor suggested by Hero999 is a good idea, even though you will lose a little voltage across it. If you can get a DC-Dc converter that has +- 15V rails it will be better, though 18V and some 15V regulators will work better.Most famous is the NE5532, which has a very poor negative rail PSRR, and if you have a high impedance in that rail it will oscillate at around 1MHz.

author: smbaker
author_rank: Regular Contributor
timestamp: September 27, 2015, 09:26:24 pm
content: Quote from: AcHmed99 on September 27, 2015, 08:13:43 pmThanks, this is one of the things I was considering. I just need to get my hands on the right zeners.Quote from: Hero9999I'm using the op-amps for two purposes. The first op-amp is a summing amplifier for a 0-10VDC signal. This part I can easily satisfy with a single supply. The second is to a buffer a 1.4 Vpp AC signal that's centered about ground. That's where the dual supply comes in handy. I don't have the schematic in a form I can easily attach. It's a derivation of the RF generator posted in the June 2014 issue of Nuts and Volts. Basically the 0-10VDC op-amp is used to drive a varactor for the voltage controlled oscillator, and the 1.4 Vpp op-amp is used for AM modulation.I originally built the contraption using a single-supply op-amp, and then did a /facepalm when I realized I needed the dual supply if I wanted to support AM modulation.

author: smbaker
author_rank: Regular Contributor
timestamp: September 27, 2015, 09:31:48 pm
content: Maybe I could do something like this:http://www.linear.com/solutions/4515to shift my 1.4 Vpp input far enough up that I could use the single-supply op-amp?Update: Wow, is this even simpler than I thought? If I follow an ac coupling capacitor with a voltage divider (for example,http://www.analog.com/library/analogDialogue/archives/35-02/avoiding/Fig1.gif), that's sufficient to level-shift my ground-centered signal up to a Vcc/2-centered signal, which a single-supply op-amp can then deal with? That would be a great simplification from what I have now.

author: Zero999
author_rank: Super Contributor
timestamp: September 27, 2015, 10:05:07 pm
content: Yes that will work but you need an op-amp which can work with the inputs down to 0V. The LM358 is a common one but there are better alternatives available.If the value of the DC component is unimportant for the AC signal, just bias the input at half the supply voltage and AC couple the signal.

author: smbaker
author_rank: Regular Contributor
timestamp: September 27, 2015, 10:08:22 pm
content: Quote from: Hero999 on September 27, 2015, 10:05:07 pmWhat would you suggest as a substitute? Preferably something that is pin-compatible (and through-hole).Thanks,Scott

author: Zero999
author_rank: Super Contributor
timestamp: September 27, 2015, 10:20:00 pm
content: The TS272/274 is my favourite one but as I said, there are many.http://www.st.com/web/en/resource/technical/document/datasheet/CD00000495.pdf

author: Richard Crowley
author_rank: Super Contributor
timestamp: September 27, 2015, 10:35:20 pm
content: Quote from: smbaker on September 27, 2015, 09:26:24 pmIf you don't need bipolar supply for your DC summing circuit, you certainly don't need it for AM modulation. Without looking at the circuit, I still have significant doubts why you really need bipolar power and DC coupling for such a simple AC application.

author: smbaker
author_rank: Regular Contributor
timestamp: September 27, 2015, 10:57:28 pm
content: Quote from: Hero999 on September 27, 2015, 10:20:00 pmI think I have some TLC2272 on hand. Reasonable substitution? Its data sheet claims it to be a 'performance upgrade for TS272, TS274, TLC272, and TLC274'.I'm excited about this and might try the substitution tonight. One of the reasons for this project was to gain some learning experience with analog circuits (I'm mostly a digital guy), and I feel I've learned some important things!

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 28, 2015, 06:17:46 am
content: use lm317 and lm337 after the dc-dc. my 2cnts...

author: mikerj
author_rank: Super Contributor
timestamp: September 28, 2015, 07:09:13 am
content: Quote from: smbaker on September 27, 2015, 07:06:34 pmIf you mean a +-10v output, then you won't even be able to do that with a TL072. The outputs on these struggle to get anywhere near the supply rails, you'll probably get no better than +-9v with +-12v supply.

author: Zero999
author_rank: Super Contributor
timestamp: October 01, 2015, 09:05:29 am
content: Quote from: SeanB on September 27, 2015, 08:00:07 pmI don't see how it makes any difference whether the ripple is on the positive or negative rail on the op-amp. The op-amp doesn't have a 0V pin. All it sees is the difference between the positive and negative supply rails, so it doesn't know whether the ripple is on the positive or negative side or the supply.

author: mikerj
author_rank: Super Contributor
timestamp: October 01, 2015, 11:03:25 am
content: Quote from: Hero999 on October 01, 2015, 09:05:29 amInput voltages are normally referenced to 0v however, so the input pins are effectively the 0v connection.

author: jitter
author_rank: Frequent Contributor
timestamp: October 02, 2015, 03:50:59 pm
content: Quote from: smbaker on September 27, 2015, 07:06:34 pmHow about using a DC/DC converter with a clean output?In measuring instruments, we often see DC/DC-converters by Traco Power (e.g. this one). These typically have a ripple (20 MHz bandwidth) of only 50 mVpp max. (some even better) without any help from external components. The opamp's PSRR will do the rest.

author: Zero999
author_rank: Super Contributor
timestamp: October 02, 2015, 05:10:56 pm
content: Quote from: jitter on October 02, 2015, 03:50:59 pmWhat's the PSRR at higher frequencies? I've noticed the datasheets for most op-amps don't specify the frequency the PSRR was measured at. Presumably the tests are conducted at DC. I'd imagine it gets worse, as frequencies increase, which is something you need to bear in mind when dealing with switched mode converters.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 02, 2015, 05:47:11 pm
content: For the high frequency ripple a passive LC filter is a good idea - up to the point of using a higher order cauer type filter. At more than 100 kHz most LDOs and OPs don't offer very much PSRR. Starting with a low noise DCDC converter is a good idea.

author: jitter
author_rank: Frequent Contributor
timestamp: October 02, 2015, 07:16:17 pm
content: Quote from: Hero999 on October 02, 2015, 05:10:56 pmAgreed, jellybean opamps may not have specs in the datasheets of PSRR vs frequency. But the opamps I see in precision instruments do have these graphs in their datasheets. I attached a few of those. Yes, they all show a decreasing PSRR with increasing frequencies, but the same goes for ripple rejection of linear regulators.My best bet to make +/- 12 V from a single 12 V rail would be a good low noise DC/DC-converter which you may improve with a low pass filter. I write may because even with "only" 20 dB PSRR at a certain frequency, there won't be much left of an already low noise. And my guess is that the switching frequency of that converter will be rather lower than in the MHz,range so that noise spec of 50 mVppmaxmay have that max (and some harmonics) at a frequency that are well within the high PSRR of 40, 60 or more dB.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 02, 2015, 08:18:48 pm
content: The DCDC converters may have a rather high frequency. Also there are often higher harmonics. Usually the high frequencies (e.g. >100 kHz) are efficiently damped by an LC filter, while low frequencies are damped by a volatage regulator. Depending on the output capacitor some LDOs may provide quite some attenuation at high frequencies. So it's worth looking for the right type of regulator. At the higher frequencies the layout gets important too.

author: jitter
author_rank: Frequent Contributor
timestamp: October 03, 2015, 04:03:44 am
content: I have access to some of those Trace Power DC/DC-converters, so it might be interesting to see what their outputs look like on a scope.That'll have to wait for next week, though.

author: jitter
author_rank: Frequent Contributor
timestamp: October 07, 2015, 01:24:29 pm
content: Did a very quick measurement of a TEN 8-2423 DC/DC-converter by Traco Power mounted on a breadboard.18-36 V in, +/-15 V (267 mA max.) out.Measurement taken without any load or external components on it, 24 VDC input. Switching frequency is 300 kHz (also spec'd in datasheet).I set the scope in high res acquisition mode and 20 MHz bandwidth limit. Just used the clip on earth lead of the probe, so picked up noise will be a bit higher.In the design in which these converters are used (measurement equipment up to 32 MHz bandwidth), 10 uH in series + 2u2//100n to GND are filtering the outputs.

author: macboy
author_rank: Super Contributor
timestamp: October 07, 2015, 09:11:02 pm
content: Quote from: Hero999 on September 27, 2015, 07:33:25 pmThis is sometimes called a capacitance multiplier. The ripple rejection at its output is equal to the capacitor at the base times the AC gain of the transistor. When using it with higher output currents you need to modify it for more voltage drop. Note that as the gain of the transistor drops with higher frequency, so does the ripple rejection.