Powering opamp using boosts - Page 1
Forum section: electronics / projects

author: raff5184
author_rank: Regular Contributor
timestamp: September 24, 2020, 11:14:35 pm
content: HiI have an AD826 opamphttps://www.analog.com/media/en/technical-documentation/data-sheets/AD826.pdfin basic non inverting configuration (gain = 10x) and double rail supply. I want to power the opamp close to the maximum supply (+-17.5V).The supply comes from a 4V voltage of a lab DC power supply (so I have no current limitations) that I boost using two LT8330, one for the +17.5V supply the other for the negative -17.5Vhttps://www.analog.com/media/en/technical-documentation/data-sheets/LT8330.pdfNow, all my components work properly independently. The opamp amplifies with a gain of 10, the boosts create the +17.5V and -17.5V BUT when I try to power the opamp from the two boosts, it doesn’t amplifies. I measured the current absorbed by the opamp, this is what I noticed: when powerd from the large DC supply it absorbs 20-30mA and works fine. When I use the boosts the current in each rail is of 7mA.I tried to use only one LT8330 to produce 35V and a virtual ground but I have the same result, the current absorbed by the opamp is 15mA. It looks like when I use the boosts, or a single one, the total current absorbed by the opamp is 15mA.What could be the problem? Mismatch/impedance issue?I'm attaching a simple block diagram, I'll upload more details if neededthank you

author: ludzinc
author_rank: Unknown
timestamp: September 25, 2020, 01:00:00 am
content: What inductor did you use? Can you test how much current your boost circuits can supply?

author: raff5184
author_rank: Regular Contributor
timestamp: September 25, 2020, 02:24:43 pm
content: Quote from: ludzinc on September 25, 2020, 01:00:00 amI tried different values from 6.4microH to 10microH with very little difference. The currents, as reported in my initial post are about +7mA and -7mA if I use two boosts each giving 17.5V, or 15mA if I use one supplying 35V. If you mean the maximum they can supply, I don't know.

author: raff5184
author_rank: Regular Contributor
timestamp: September 25, 2020, 02:27:54 pm
content: Quote from: blueskull on September 25, 2020, 01:39:14 amBecause I didn't know about this circuitFew questions:1. Would it provide the current I need for the opamp though (20-30mA)?2. Why do I need rectifiers if my supply source is DC? Is it to rectify the inductor output?3. The MT3608 has a max output of 28V while I need a rail-to-rail of 35V (+-17.5V), would this circuit still work?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: September 25, 2020, 04:19:57 pm
content: Quote from: raff5184 on September 24, 2020, 11:14:35 pmHi,can you share your full schematic?Jay_Diddy_B

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: September 25, 2020, 04:56:33 pm
content: Quote from: Jay_Diddy_B on September 25, 2020, 04:19:57 pmYes, in particular there needs to be a shared ground for that opamp to get +-17.5 volts. Suspiciously not shown in block diagram.

author: raff5184
author_rank: Regular Contributor
timestamp: September 25, 2020, 05:10:54 pm
content: Quote from: RandallMcRee on September 25, 2020, 04:56:33 pmsure I will share the schematic.The GND is not shown in the schematic, but yes the supply source GND is shared with the two boosts and the opamp. The input signal to the opamp is relative to the GND

author: KT88
author_rank: Frequent Contributor
timestamp: September 25, 2020, 05:11:36 pm
content: can you share your full schematic?...and layout as well.The AD826 like other fast amplifiers requires low resistance feedback (<1kOhm) as well as very! low inductance of the feedback loop. Note that bypass capacitors ar part of the loop and contribute to the overall loop inductance.Strange current consuption often comes from oscillations of the amp which can be subtle in amplidude and might go unnoticed if one doesn't look for it e.g. BW limitation of the scope...Residual noise from the boosts could possibly excite some oscillations.For the AD826 about 15mA is what one would expect with 0V input. In addition there would be the current through the FB resistors with a signal present.7mA is definitely odd...CheersAndreas

author: raff5184
author_rank: Regular Contributor
timestamp: September 25, 2020, 06:48:43 pm
content: this is the circuit,note that C8 is with the + towards the GND, differently the boost doesn't work.As for the OpAmp, I just need to amplify the 3Vpp in input, nothing accurate o preciseThank you

author: KT88
author_rank: Frequent Contributor
timestamp: September 25, 2020, 08:22:44 pm
content: Two issues:The bypassing is not sufficient: you need a small package (0603) 3.3uF or more ceramic cap close to the amp.about 1kOhm in the feedback max. 10k won't work - too much phase shift agaist the input capacitance...Tip: you could use only one regulator: The inverter can be attached to the non-inverting boost.

author: raff5184
author_rank: Regular Contributor
timestamp: September 25, 2020, 10:56:37 pm
content: Quote from: KT88 on September 25, 2020, 08:22:44 pmI did both things, smaller resistors (820and 82) and larger bypassing. Still doesn't work. The opamp works totally fine if I power it from the DC supply. I also tried a different boost, single rail at +27V with a virtual ground to generate the negative supply, still nothing. The boosts output voltages are correct, BUT when I connect the opamp, they go down to 3-4V

author: KT88
author_rank: Frequent Contributor
timestamp: September 25, 2020, 11:00:56 pm
content: Could you provide the layout? A lot can go wrong with that as well...

author: raff5184
author_rank: Regular Contributor
timestamp: September 25, 2020, 11:04:26 pm
content: I don't have it. I have everything on breadboards

author: KT88
author_rank: Frequent Contributor
timestamp: September 25, 2020, 11:28:33 pm
content: That's very likely your problem. With a linear supply it might be borderline stable. But the noise of the boost pushes it over the edge.On a breadboard you have no realistic chance to get a boost output clean enough. When you look at a boost converter the L/C lowpass is pointing towards the input - the output gets all the switching noise (it is the other way around for a buck).You also won't get the parasitic inductancies low enough arond the opamp. That way it can't work properly. Instead of a breadboard you could try a 'dead bug' design which allows to have a solid ground plane. But it sill requires a lot of skills to pull that off.The best way to prototype it would be a PCB. They got quite cheap over the last years. JLCPCB seems to be fast and good in quality.Once you have a layout ready I would suggest to post it in this thread for review before you place your order.Btw. - did you terminate the unused amp (buffer config, +input to GND)?CheersAndreas

author: raff5184
author_rank: Regular Contributor
timestamp: September 25, 2020, 11:32:40 pm
content: Thanks, I'll see what I can do.I implemented the changes suggested, I did terminate the other opamp. The opamp works with small input values up to 400mV, amplifies up to 4Vpp (equal to the supply range). After that it saturates. Again the saturation only happens with the boost, with lab DC supply still works fine.I tried to dead-bug the boost only, no changes

author: julian1
author_rank: Frequent Contributor
timestamp: September 26, 2020, 08:41:24 pm
content: The LT8330 datasheet p20 shows a couple of inverting circuits. Which one did you use to create the negative rail?Are you able to load both +ve and -ve rails simultaneously, and then check with a multimeter that +ve, gnd -ve all show expected voltages?

author: raff5184
author_rank: Regular Contributor
timestamp: September 26, 2020, 09:07:59 pm
content: Quote from: julian1 on September 26, 2020, 08:41:24 pmI used the second one but with different R1 and R2 and no R3 and R4 as sowed in my circuit.All the boost voltages are correct AS FAR AS I don't connect any load. The moment I connect the opamp they drop from 17.5 to 3-5V

author: julian1
author_rank: Frequent Contributor
timestamp: September 26, 2020, 09:18:41 pm
content: So you drop R3/R4 and just pull EN/UVLO high, to disable UVLO? sounds ok.Disable/disconnect/remove the op-amp. And just load the boost outputs with resistors - for 1mA, and 20mA. And see if the supplies behave.Do you have a scope. I would not be confident trying to do sepic like converters (the inverting one) without the ability to probe them.

author: raff5184
author_rank: Regular Contributor
timestamp: September 28, 2020, 06:27:33 pm
content: Quote from: julian1 on September 26, 2020, 09:18:41 pmYes the part about R3/R4 is correct.This is what happens.I use R=15k, and it works fine. I measure about 1mA (at 17.5V boosted output on open circuit), while with the R inserted the boost output is 13V.Different story if I try to drive 20mA. I use R=1k so I expect 17mA. But I only measure 3mA and the boost output across the load is 3V (which makes sense 3V/1k = 1mA).So the boost can't drive lower impedance load? BUT what I don't understand is that: if a single boost can drive 15mA, when I use 2 boosts I should be able to drive 20mA

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: September 28, 2020, 06:42:27 pm
content: Hi,what kind of capacitors are you using for C3 and C8?Regards,Jay_Diddy_B

author: raff5184
author_rank: Regular Contributor
timestamp: September 28, 2020, 06:51:10 pm
content: I tried both electrolytic and tantalum, no difference

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: September 28, 2020, 07:30:50 pm
content: Hi,You need ceramic.The ADI demo board for the LT8330 uses ceramic.Link:https://www.analog.com/media/en/dsp-documentation/evaluation-kit-manuals/DC2018AF.PDFJay_Diddy_B

author: raff5184
author_rank: Regular Contributor
timestamp: October 02, 2020, 12:04:58 am
content: Hi,I'm still stuck on this design. I used ceramic caps, a little better but not much improvement.At lest now the amplifier is powered and I see a clear output, but it is not amplified. The output p2p voltage is essentially my supply amplitude, as if the boost/s are not there