Opamp and supply choice - help needed

[oliglaser]

Hi all,

I'm designing a PIC32 based USB oscilloscope. I need a dual supply for the opamps as it simplifies input impedance and level shifting. I selected a unregulated +5V to +/-5V DC to DC converter (needs to be isolated so this limits my choices here) and then I was either going to use +/-3V LDOs in front of this to provide the supply for the opamps, or just filter the +/-5V instead.

The problems I have with the +/-3V idea are finding a suitable hi-speed, fet based opamp that works with the supply. I have found one or two possibilities but neither are ideal, one is right at the bottom of the supply range (5-24V) and does not have rail to rail inputs), and the other would have to be used outside recommended supply range.

The problem I have with the +/-5V idea is I am worried about the ripple on the supply, and whther it will affect the ADC and opamps that much. The ADC is only 8-bit, so I am thinking I am worrying too much here, but as I am no expert in these matters I would like to hear any opinions. Also what filtering technique would be best?

So the questions are:

1. Which option do you think is best?

2. How would you implement it?

Thanks in advance..

 

[srizbf]

ti and linear technolgy (and possibly analog)
may have lowvoltage rail to rail opamps.

srizbf
11thmay2010

 

[fcfusion]

Ripple is a very important factor in your project. DC-DC converters usually create considerable ripple (several milivolts, sometimes even more) so, your entire circuit should have a very stable power supply.

IMHO, if your going to use DC-DC's you should power your Opamps and PIC32 through LDO's with good input and output filtering, to reduce DC-DC's ripple. There is chance you still have ripple because of EMI.

About choosing the Opamp, you should check www.analog.com. They have a very wide selection of opamps, you'll find what you need there.

 

[oliglaser]

Thanks for the replies.

I have already looked at the sites mentioned, and up to now have not found anything (although I may be missing stuff)

The DC to DC is unregulated, the ripple is around 50mV, so I was thinking LDOs with filter caps and choke. The trouble is I need the voltage headroom for the LDOs and then I run into problems finding an suitable opamp that will run of the +/-3V supply.

If I just use a choke and caps on the +/-5V supply, it won't be as clean, but I can find a suitable opamp. I may have to do this if I can't find anything, although I won't feel too comfortable about it.

I will carry on looking, but if anyone has knowledge of a high speed, low voltage, high input impedance (>30Megaohm) rail to rail opamp I would be very interested.