"Do-It-All" Box - Page 1
Forum section: electronics / projects

author: Jeffbeck67
author_rank: Newbie
timestamp: September 11, 2020, 01:36:05 am
content: Hi Everyone,I wanted to do something since a long long time: A small box that contains all daily feature we need. Ideally I would fit in a Hammond box No need for top notch precision, just things as it come.So I listed:- Differential Opamp- Low noise Amplifier (x10, x100)- x10 and x100 Opamp circuit- 10Vref- 1/10 and 1/100 resistor bridge (max 1kV)- 100uA constant current sourceAttached is a preliminary circuit.I realized it's likely ambitious to fit everything in a small Hammond box. There is already 13 Banana!I should be battery powered/usb charging capable.Any opinion ?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 11, 2020, 03:30:40 am
content: Is there a question in here?You've connected the current sink wrong. The inverting input of U3A should just use the voltage over the shunt resistor as reference.Test the sub-circuits on a breadboard before you order PCB's. It will catch such errors as mentioned above, and it's also much easier to tweak the circuit for stability, add a capacitor here or there and such.Maybe you can make the thing smaller by replacing most of the banana jacks with a multi position rotary switch.Where does your +15V and -15V come from? I see no power supply section, nor do I see connectors for it.Using 100nF decoupling caps is fine and all, but also add some bulk capacitors, of size 10uF to 100uF, also depends on how you make the power supply.NE5532 has a worst case offset voltage of 4mV. If you amplify that with 100, you have almost half a volt at he output without an input signal. Is that acceptable?Consider a opamp with lower offset voltage, or use a trimmer for nulling the offset.If battery fed, current consumption may be an important factor, so have a look at the current consumption of your circuit.When you add connectors to the mad and angry outside world you need protection for ESD, EMI and that sort of stuff. When you add (long) cables you also add capacitance, and capacitance on the output of opamps may introduce instability and cause oscillation.On a box with so many sub circuits it may be usefull to use 2 or even 3 banana jacks for GND. This would prevent the need of those stackable plugs. Those stackable banana plugs are useful, but it's not very convenient if you want to replace the middle plug of such a stack. Using multiple GND plugs also lets you implement star grounding.

author: Jeffbeck67
author_rank: Newbie
timestamp: September 11, 2020, 04:56:01 am
content: Hi and thanks for your inputs.Current sink: thanks I don't know what I was dreaming today.Banana #: I want to have something where IE the diff amp can be followed by the amplifier thus making these plugs needed.Power: yes, it will be bat. operated. I first want to see what does all fit in a box.NE5532 offset: Good point! TBH I never played with high gain amps.EMC: Note 4 mounting hole that will be connected to GND and casing.

author: newbrain
author_rank: Super Contributor
timestamp: September 11, 2020, 08:36:07 am
content: Quote from: Jeffbeck67 on September 11, 2020, 04:56:01 amKeep in mind that (after the correction) the 100 µA sink will have a very bad compliance range, as there are (by definition) 10 V across the 100 kΩ resistor.You'd better reduce the reference voltage to the + op-amp input to something more useful (e.g. 0.5 V or even less) and use a suitable resistor.

author: magic
author_rank: Super Contributor
timestamp: September 11, 2020, 09:26:50 am
content: Put some 50~100Ω in series with opamp outputs to isolate them from capacitance of external cables. May oscillate otherwise.NE5534 has offset trim and more open loop gain. LM4562 is a dual with similar OLG at audio frequencies and not much more expensive. Silicon lottery is another solution for a one-off build - buy a few chips and hope that one will be good enough. Modern production parts rarely hit their worst case limits.Johnson noise is a consideration. 470Ω isn't too bad for the 5532, but it will compromise the 5534 or 4562 a bit. You could reduce feedback network resistance, both of the above chips will drive 2kΩ or even 1kΩ with only minor loss of performance.

author: Jeffbeck67
author_rank: Newbie
timestamp: September 12, 2020, 04:25:17 am
content: Thanks for all these inputs.I have hundred of 5532 since they cost nothing. of course one might later go for higher specs opamps.

author: magic
author_rank: Super Contributor
timestamp: September 12, 2020, 07:23:46 am
content: So just pick one with low offset, problem solved.Quote from: Jeffbeck67 on September 12, 2020, 04:25:17 amWait, are they NE5532 or fakes from AliBay?

author: OM222O
author_rank: Frequent Contributor
timestamp: September 12, 2020, 09:03:44 am
content: Quote from: magic on September 12, 2020, 07:23:46 ammouser has them for less than 0.40$ at quantity of 1!I think that counts towards "nothing" compared to some DACs, ADCs and voltage references that cost upwards of 20$ per chip.