We can do this from USB as well if we choose to. +-5V was in the orignal design but was removed because it wasn't nessasry. Now we have found out it might be a good idea anyway, we can just implement it again.simce said:I'm focusing on input amplifier. If we use other power supply we can deliver +/-5V on opamps that have better characteristics.
FEATURES
UNITY-GAIN BANDWIDTH: 450MHz
WIDE BANDWIDTH: 200MHz GBW
HIGH SLEW RATE: 360V/µs
LOW NOISE: 5.8nV/√Hz
EXCELLENT VIDEO PERFORMANCE:
DIFF GAIN: 0.02%, DIFF PHASE: 0.05°
0.1dB GAIN FLATNESS: 75MHz
INPUT RANGE INCLUDES GROUND
RAIL-TO-RAIL OUTPUT (within 100mV)
LOW INPUT BIAS CURRENT: 3pA
LOW SHUTDOWN CURRENT: 3.4µA
ENABLE/DISABLE TIME: 100ns/30ns
THERMAL SHUTDOWN
SINGLE-SUPPLY OPERATING RANGE: 2.5V to 5.5V
MicroSIZE PACKAGES
Requirements for Input Signal Conditioning:
Designing front-end for a DSO is difficult. Here are some of the problems a designer faces:
1/ A 500MHz scope needs 1GHz of bandwidth with a clean step response.
2/ This BW must be obtained with 1MOhm input impedance!!
3/ No resonance's of components should be present up to about 5 x the BW (5GHz)
4/ Because of high input resistance and low capacitance, you can't use transmission lines (the way to handle high frequencies)
5/ You need to keep the components and layout very small
6/ This delicate circuitry needs to withstand high electrostatic discharges and input voltages.
7/ Most difficult is to have gain flatness from DC to some part of the operating BW
I would suggest to use OPA657 or OPA656 and create +-5V from the 3.3V supply. Then we don't need the -3.3V supply anymore.monnoliv said:We're looking now for the point 7. Can you help us?
This is what i was talking about. Are there samples for these chips?I would suggest to use OPA657 or OPA656 and create +-5V from the 3.3V supply. Then we don't need the -3.3V supply anymore.
OPA657 and OPA656 looks like the best opams for the purpose found so far.
Just click at "Request Samples" here:simce said:This is what i was talking about. Are there samples for these chips?I would suggest to use OPA657 or OPA656 and create +-5V from the 3.3V supply. Then we don't need the -3.3V supply anymore.
OPA657 and OPA656 looks like the best opams for the purpose found so far.
monnoliv said:We're looking now for the point 7. Can you help us?
I only have experience with designing input stages with split path using LF opamp to handle the DC conditions and FET stage to handle the HF-AC part. The 2 signal paths are then combined to give a smooth transition from DC to AC. This was the way input stages were designed before very high BW opamps came along. It had a few advantages. I have included a typical design just for interest.
I will observe and comment when I think it will be of any help.
monnoliv said:Thank you for your contribution.
But I can't see the advantage of splitting AC and DC part. Do you have a higher input voltage range with this circuit ? Don't you need a ladder (attenuator) ? What about output impedance ?
Regards,
But I can already see an input impedance of 1Mohm in your circut (2*500kohm). Did you have more inpedance in front of this circuit?E-design said:The circuit is only the impedance converter stage. I have not drawn in the attenuators before this.
E-design said:One thing I have not noticed in your guys design is a BW limit function. This is a very important feature. (maybe I missed it in the discussion)
ME said:Have you been working at Tektronix E-design?
ThanksE-design said:Normally the BW limit is at 1/5th of the full BW. I will do the limiting after the input stage where you have lower impedances to work with
All these IC have an input common range of -Vcc to +Vcc - 3 !!!What do you think of these op amps as input buffers?
We wait impatiently... :wink:I will make more drawings when I have more time. For those who still follow...
Ok but as Me ask, is a one order filter is enough ?I will do the limiting after the input stage where you have lower impedances to work with
The sampling frequency is 100MS/s.Is the 50MHz repetitive sampling?
I presume that it's implemented in the analog part. Is there a simple mean (and cheap) to realize this ?You need also an anti aliasing detector to know when you are trying to sample a input frequency that is too high.
We need large storing memory to do this, for the moment it's accepted (but not fixed) that the storing memory is 1024x10x2 bits. The Idea to have external SRAM is interesting... and on the way (no?)One common problem is capturing an fast event on a slow sweep speed. Then you need to employ peak detect. Here is a link with a short description that saves me a lot of typing.
The software is not an issue from my point of vue, the hardware is more complicated.It is also good to have an average function when making measurements on noisy signals. This can be implemented in SW later.
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