single supply oscilloscope input help

[oliglaser]

Hi all,

Specs and Goals

Input Voltage ~20V DC/AC coupled
Input Impedance = 1 megaohm
Supply Voltage = +5V
ADC range = 1.5 to 3.5V

I am trying to design an oscilloscope input using a single supply opamp (+5v) so I need to shift the input common mode voltage to 2.5V while dividing it by 10. I have no problems doing this either with inverted or non-inverted design, but keeping the input impedance at 1 Megaohm for DC-20MHz is not as easy - if I use a reference voltage then the AC sees a lower impedance with a non inverting setup, and if I use an inverting setup then the impedance changes with the input voltage due to the voltage shifting. Can anybody advise me on this? Do I have to compromise or is there a better way to proceed?
Any help would be most appreciated.

 

[keith1200rs]

Oscilloscope front ends normally use small relays and resistive attenuators to scale the signal before then going into a high impedance buffer. You design the attenuator to maintain a constant 1M ohm and also include the AC coupling there. You need some overvoltage protection into the buffer. Then do the level shifting.

Keith.

 

[karesz]

Hi,
Morover is in my opinium a single supply amp for scope inputs some falsh thing...
You must so & so have +/- supply in the system, apply that here & you will have a simpler DC.shift setting too!
K.

 

[oliglaser]

Thanks for the replies.

Then do the level shifting.

With a single supply I need to shift the input level at the first opamp to allow for negative swings - unless there is another way round this problem?
My problem is maintaining a constant 1Meg impedance over a wide frequency range whilst dividing and shifting the input voltage as the ac sees the power to the reference voltage as ground. If it is possible to maintain exactly 1Meg - can anyone give me an example of how this would be done?

 

[karesz]

You see;
you didnt gave knowed your system design!
We see only a small portion as the preamps up to ADC_its not serious to answer & wait for that as some exacter stuff...Sorry.
I must assume that you dont wish to build a tube/screan scope?
Some circuits are not to see if you can/dont wish give more details.
K.

Added after 2 minutes:

As Keith sayed it for you too:
A scope has to begin (as a SA, & Modulations Analyzer... too) with an attenuator (& AC/DC coupling) + switcher matrix!
So can you have the constant imput impedance, all other are dreams, & unneede complications-sorry!.-)

 

[Borber]

You may use P-Channel MOSFET like BSS84 as input follower but drift due to temperature and linearity can be a problem while passband reaches 100MHz. Source resistor must be greater than 1kohm.

 

[oliglaser]

Karesz, sorry I can't quite understand what you are saying exactly though I get parts of it - are you saying it can't be done this way? The ADC input is 1.5V to 3.5V but I am just interested in the first couple of buffers, the dividing and shifting etc - though I can draw a more detailed circuit for you if this is what you are saying.
I figure since there are PC Scopes that run off USB power out there then there must be a reasonably satisfactory way of doing this using single supply opamps or possibly a FET as borber said - does anyone know how they do it?. I can manage to attenuate the signal and keep it within 100K of 1 Megaohm from DC to 100MHz+ using the design above, but would really like to keep it steady if possible.

I am using a PIC32 and have already built a couple of prototypes that communicate with the PC via USB - this works okay so now I am getting round to building the analogue input and would like to have it all powered from the USB. Sample rate will be around 20Msps, though I may use a PLD and go for up to 200Msps+. I'm in the middle of another project at the moment so this is quite slow going - still just trying things out really.

 

[oliglaser]

Still in search of an ideal solution if anyone can help?

 

[keith1200rs]

Either

1. Use a negative supply (easy enough to generate) or

2. Have a separate diffamp for each gain range as well as resistor ratio switching.

The problem with the single supply is that to level shift a negative signal requires a potential divider which loads the signal. While that can be done, when you switch the front end attenuator the output impedance of the attenuator changes. This impedance must be taken into account in the design of the attenuator and level shifter. Normally the attenuator would feed a high impedance input, but that requires a circuit that can handle negative voltages.

So, I think you could do something with a single supply if you have separate diffamps (and maybe attenuators) for each gain range, but it is a lot more complicated than adding a simple DC-DC converter or charge pump to generate a negative voltage for a buffer. The current required would be tiny as it would only need to power one opamp. The buffered output can then easily be level shifted.

Keith

 

[Borber]

For start this circuit can help. In temp range of 0-70°C output voltage shift is abt 50mV.

 

[oliglaser]

Thanks for all the replies, I haven't looked at this for a while due to other priorities but I'm at the stage where I will be using it now.

Keith1200rs - I think the DC to DC converter probably sounds like the best option at the moment so I will have a look at this, I was trying to avoid any such complications but it's a lot simpler than designing for different gain ranges as you say.

Borber - thanks for the circuit, I will test it out and see how it goes.