Opamp and supply choice - help needed

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..

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

srizbf
11thmay2010

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.

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.