+5VDC to -5VDC

[leonel]

I need a basic help! (I think...)
I have a regulated +5VDC and i want a -5VDC to power a opamp. What's the best scheme?

 

[Nigel Goodwin]

I need a basic help! (I think...)
I have a regulated +5VDC and i want a -5VDC to power a opamp. What's the best scheme?

Try the hardware for my PIC analogue tutorial, where I do just that!.

 

[leonel]

To be honest... i was looking in your site and i didn´t find.
Can you put me a link or tell in what section it is?

 

[electronic mind]

if u need the -5vDC

all u need is to use an IC ICL 7660 produced by Intersil
it will inverse its input in a range of about (1.5 v - 10 v)
and i used it to power up the op-amp in one of my circuits

 

[Nigel Goodwin]

To be honest... i was looking in your site and i didn´t find.
Can you put me a link or tell in what section it is?

Here , it uses the same 7660 that 'electronic mind' suggested.

 

[leonel]

Ok thanks...
Why i need that?
- If i don't power my opamp (LM324N) with Vcc=+5V and Vee=-5V my current and my voltage gives me a value that it's not supposed to be...
For example, if R1=1K and R2=150R using the inverting configuration Vcc=+5V and Vee=GND, i do not have Vout=(R2/R1)*Vin.
Can anybody tell what's happening?
Regards

 

[Nigel Goodwin]

Ok thanks...
Why i need that?
- If i don't power my opamp (LM324N) with Vcc=+5V and Vee=-5V my current and my voltage gives me a value that it's not supposed to be...
For example, if R1=1K and R2=150R using the inverting configuration Vcc=+5V and Vee=GND, i do not have Vout=(R2/R1)*Vin.
Can anybody tell what's happening?
Regards

It's simply what your reference point is, by using a split supply your reference point becomes zero volts, if you use a single supply it's usual to have your reference point at half of the supply - essentially the same as using a split supply!.

As a simple example:

1)
You have a single 5V supply opamp, referenced to zero volts.

It's set as a unity gain inverting amplifier.

The input is +1V, so the output 'should' be -1V, but there's no way it can as there isn't a negative supply to provide the voltage.

2)

You have a single 5V supply opamp, referenced to 2.5V (half the supply).

It's set as a unity gain inverting amplifier.

The input is +3.5V (+1V above reference), so the output will be 1.5V (-1V below reference), in this way the amplifier works just the same - BUT the inputs and outputs aren't referenced to zero volts.

 

[leonel]

To be honest i don't understand very well, but as far i understand i cannot supply my opamp with Vcc=+5V and Vee=GND using the inverting configuration!!!? Or it's possible and have good results?

 

[Nigel Goodwin]

To be honest i don't understand very well, but as far i understand i cannot supply my opamp with Vcc=+5V and Vee=GND using the inverting configuration!!!? Or it's possible and have good results?

You can do it, but with various restrictions, we really need to know EXACTLY what you are trying to do!.

 

[leonel]

I'm doing the inverting configuration to simulate 4-20mA output pressure sensor and have a voltage output (it's a I-V converter). My ideia it's have a final configuration like the attatched file (fig 2), but i do not have my pressure sensor yet, so with a trimmer i'm simulating an input 4-20mA and measure my output voltage (fig 1). But as i said i do not have correct values in fig 1!!? I'm affraid that when i do my pcb (with the attatched configuration fig2) i do not correct values to measure with my PIC.

 

[audioguru]

2 serious problems:
1) You have the reference input (the opamp's + input) connected to 0V. With a single supply it is supposed to be connected to half the supply voltage. Use a voltage divider with two 10k resistors to make the half-supply voltage divider. Then the reference input will be +2.5V.
2) You used an MC1458 opamp that needs a total supply voltage at least about 10V. With a supply voltage only 5V its output will swing less than 2V.
The LM324 or LM358 operate with a total supply voltage down to 3V. With a 5V supply their output will swing about 4V.

 

[audioguru]

Oh yeah, another problem:
The 220 ohm value of the R1 feedback resistor is so low that an opamp can't supply enough output current to drive it. Most opamps operate poorly with a load and/or feedback resistor of 2k or less.

 

[eblc1388]

Hi leonel,

Would the following circuit serves your requirement?

 

[leonel]

yes, it will do!! But i'll have to use LM324 to have the output 0.8-4V. Using MC1458 i do not have correct values (problems that Audioguru pointed)!!?

 

[eblc1388]

yes, it will do!! But i'll have to use LM324 to have the output 0.8-4V. Using MC1458 i do not have correct values (problems that Audioguru pointed)!!?

Yes, problems with MC1458 is the output swing at 5V supply and the common mode input voltage with Vee=0. You need a special type of opamp(LM324/LM358) here which can accept input voltage down to zero volt while its Vee is also zero volt.

Even with LM324/LM358 one is pushing the Vout limit of 4V with 5V supply voltage. It would benefit a lot if you can supply the opamp with a higher supply voltage. There is no need to change any component value in the schematic I posted to do that besides giving it a higher supply voltage.

Perhaps others here can advise you with the part number of a "single supply rail-to-rail op amp" or you can put these in Google for a lot of hits.

 

[leonel]

Ok thanks...
I'll supply with +24VDC.
I think that´s no problem suplly my pressure sensor with the same +24VDC (it's a two wire connection: +24VDC and Iinput)!!??
Or you think i should use a transformer with two secondary output's?

 

[eblc1388]

Ok thanks...
I'll supply with +24VDC.
I think that´s no problem suplly my pressure sensor with the same +24VDC (it's a two wire connection: +24VDC and Iinput)!!??
Or you think i should use a transformer with two secondary output's?

I don't know about the details of the pressure sensor you're going to use but 24V supply, if regulated, would be OK for LM324/LM358 although it is a bit on the high side. Check to see whether the pressure sensor would work with minimum +12V or more(which is highly unlikely). If so, use a 7815 voltage regulator for both the pressure sensor and LM358 supply. Alternatively you can use the 7815 just for the LM358 supply only with the pressure sensor using +24V. You do not have to use a transformer with two secondary windings.

The pressure sensor is designed to output a constant current proportional to the pressure so it is correct to connect the sensor to +V and input. I have added protection diodes to opamp input and output to prevent possible damage to opamp and PIC.

 

[audioguru]

+ 24V is fine to power the opamp. The max allowed for most opamps is +36V and some are rated for +44V.

Is the old LM324 or LM358 fast enough? They were the first opamps designed for low supply power so their outputs are darn slow.

The newer MC33171 single, MC33172 dual or MC33174 quad opamps are also low power but much faster. The MC34071, MC34072 or MC34074 are very fast but use the ordinary power of a couple of mA per opamp. They all have the same wide input and output swings as the old LM324 and LM358. :lol:

 

[eblc1388]

There is still one point which should be discussed.

Right now the Opamp is having a gain of two, providing 4V to the PIC for A/D conversion.

How about removing the Opamp altogether and just use a 200 Ohms resistor at the input of the PIC?

Question is: What extra advantage does the opamp circuit offers?

 

[leonel]

In the beginning i thought in your ideia, but i think i'll have problems with impedance!!??? That's why i thought one solution with opamps...