Two more problems: if the amp rails, it can cave in the power supply, maybe latching things up. Or it might turn off Q2 through the b-c junction of Q1.
A fet for Q1 helps. A resistor between Q1 and Q2 fixes the other problem.
It's almost appealing to go back to discretes, especially for a fast differential version.
and if we wanted to start putting some numbers in there, we'd start with the shunt and the load.
Since you're looking for low millivolts out of the shunt let's assume the entire 250V is dropped across the load. Then with your specified 10mA max current into the load, it'll look like:
E 250V R = --- = ------- = 25000 ohms I 0.01A
Now, assuming "low millivolts" means 10mV with 10mA through the shunt and the load, that makes the shunt resistance:
Now, assume you've got a rail-to-rail input opamp which you can drive from a 25V supply and which has a common mode range from 0V to the supply voltage. Then its inputs will have to be slightly below
25V and to get there you'll need to drop the 250V to --+--[1R]--+--[25kR]--+ | | | R1[226k] [226k]R3 | | | | E2--++--E4 | | | | R2[24k9] [24k9]R4 | | | | GND>---+--------+----------+
With the 250V at zero, E2 = E4 = 0V, and the output of your instrumentation amp would be 0V, ideally.
The original post was about producing a ground referenced voltage proportional to load current with 8-bit accuracy and 1MHz bandwidth, so what has changed?
Deconvolution has little to do with the sampling theorem...
But please explain what mathematical transforms you had in mind, and what analog requirements could be relaxed, when you wrote
"This means a more intelligent approach would relax the complexity and performance requirements of the analog kluge and compensate with a deconvolution of the data."
Fred doesn't know, he just copied the verbiage from Wikipedia because it sounded intelligent ;-)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
John, I'll have to say, no, that's wrong. If you think it through carefully, you'll see those states either cannot occur, or they quick move on to a safe operating condition.
Apparently you have absolutely no clue of the analog GBW requirement of the circuit to maintain less than 1/2 LSB error at 8 bits out to 1MHz. Obviously you would want to keep things simple, something like a dominant pole low pass roll-off for that, then unless you have a DSP with FFT, you would use a time domain convolution to reconstruct the unfiltered data record of the current. Maybe this doesn't have to be done in real time, maybe the circuit can just acquire the data on both the current profile and the sensor response to a known stimulus, and this can be processed later, download the files to a PC or workstation for processing and display. I'm not real concerned about the OP and his imagined requirements, skill set, or anything else, including your opinion or ideas, you're so dull and boring...
What neither you nor I have a clue of is whether the op expects 1/2 lsb at 1 MHz, or whether 1 MHz might be an acceptable -3 dB point. In the real world (hello!) the latter would be the likely situation. Especially seeing that the latter is totally out of the league of this situation.
You can't computationally replace bits once they are lost. The key to accuracy here is doing the analog stuff (and digitization) right, not massaging the data after it's trashed. Yes, analog circuit design bores you.
On a sunny day (Wed, 12 Dec 2007 07:23:08 -0800) it happened John Larkin wrote in :
I have thought of a third way to do this. Say you use a 74HC4054 CMOS switch on the high side, that switches at about 10MHz between both ends of the shunt, and apply that 10MHz carrier, with amplitude exactly equal to the voltage drop over the shunt, via a 1:1 transformer to an opamp precision peak rectifier on the ground level. I am still thinking what to use to swith the 4053, a 555 is not fast enough. UJT?
The high-side stuff could be completely unpowered: one transformer sending a carrier square wave up, to switch a pair of small mosfets, and a second transformer to bring the modulated signal back down.
Somebody (one of my competitors, actually) does thermocouple acquisition that way. We float the entire preamp/adc per channel, with a dc/dc converter powering the floating stuff and an Analog Devices logic isolator bringing the data back down,
You don't what you're talking about. Deconvolution no more manufactures lost bits than pre-emphasis 'cheats' line loss. Deconvolving a linear circuit is the simplest possible use.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.