I'd appreciate some witty put-downs and scathing feedback on the following.
I want to take the opportunity of a circuit redesign on a piece of scientific instrumentation to use a differential output to reduce EMC problems, but our industry normally uses single-ended analog signals for historical reasons, ie they tie one input line to their circuit 0V. Does this make use of differential signals pointless? I'm using an isolated supply to power my circuit, so it's floating and will break any ground loops - I hope. I was thinking of converting my own single-ended signal to differential down the connecting cable using an ADA4922-1 from Analog Devices, which does the conversion simply.
Examples I've seen show differential-drive outputs used with twisted pair. Is there any reason not to use coax? My highest signal freq is
500kHz, so I'm thinking TP would actually be better as I understand coax is really only effective at screening higher freqs.
Our industry uses 50 ohm coax. The differential amplifier datasheets I've scanned imply they are designed for loads of 75 or 100 ohms. Are differential amps so specialised for driving ADC's that they aren't suitable for driving 50 ohm coax?
I only have to transmit analog signals of about 500kHz bandwith over 4 metres of cable. I strongly suspect that at least one of the bits of equipment I'm expected to drive has an input impedance of about 500 ohms, ie is poorly matched. It doesn't seem to have any reflection problems with the bit of kit I'm replacing, which used an AD8045 video line driver, so I guess reflections etc aren't much of an issue. Am I being naive here - for example would you expect problems driving a poorly terminated line with a diff amp rated at 100'sMHz GBW driving it?
A possible circuit gotcha is, I want to amplify down to near DC (say
10Hz) and retain the DC offset information in the output so transformers are ruled out. I don't care if the offset is shifted as long as it's constant.
What are folks' favourite ICs for single-ended to differential output conversion?
At these speeds, just two opamps, one inverting and one non-inverting. Something fast to keep the phases balanced.
The ADA4922-1 pretty much does this. Nice part, lots of swing and lots of power dissipation. Drive two coaxes with precision 50-ohm source terminator resistors. Ground the shields on both ends. Source termination reduces the dynamic load on the opamps and helps keep distortion and power dissipation down. Don't terminate the receive end.
What's your dynamic range? What's the physics? I'm doing a very similar thing, moving a wide-dynamic-range signal across a room on two coaxes, KHz to 20 MHz, and need to reject ambient RF. We're going for maximum swing (20 v p-p differential) to climb above the noise. Source-only termination gives 1.00000 gain through each cable leg, maintaining the CMRR.
I am trying to improve a photodiode amplifier. Dynamic range is about 16 bits, ie the peak signal coming out of the amplification section is up to 4V peak (always positive-going above 0V), this is presently fed down a cable to a 16 bit ADC in another box. This other box, which I can't change, would be unhappy with inputs over about 5V. I believe the circuit is likely to be re-used for other similar applications with other equipment, which would typically expect inputs up to +/-5V so should be happy with this 0 to 4V signal.
Starting point of the circuit improvement is to convert the extant preamp to Phil Hobbs' cascode circuit which I believe will reduce noise / increase bandwidth. I thought I may as well try improving the circuit in other ways, to get experience with new techniques we could use on a range of products. I've considered and dismissed a couple of tricks I figured were too high risk like a notch filter to get rid of hum (this would give phase shift as you swept across its notch freq, and some people look at signals right down to 20Hz); but I've added a 300kHz linear phase filter to reduce HF fuzz, and an isolated power supply to get rid of earth loops.
Concerning the earth loops: this market - electron microscopy - is full of weird metal-boxed instrumentation from different manufacturers which people buy and hook together with coax, without thinking about earth loops. This is usually 50 ohm coax terminated with BNC's. So, I'm not sure I can use two separate coax's for my output as I may have problems connecting it to legacy equipment - but I'll have a good think about this. I like that idea.
The monitoring equipment is placed a few metres away from the 'scopes as they are very sensitive to ambient magnetic fields. This equipment is several other boxes ending in a PC to clean up and display a picture of the sample being peered at. A typical cable run from the 'scope is 3 to
5 metres, under floorboards so you don't trip over them, plus a whole mess of interconnecting coax cables behind the PC and other boxes. This leads to ground loops when people plug things into different mains sockets around the lab... and just a small ground current in these loops can physically wobble the electron beam in the scope, or create enough interference in the high gain amplifiers, to cause noticeable banding at
60Hz.
Users are interested in the DC level of the signal (corresponding to photodiode dark current of a few nA) as well as AC signals down to around 20Hz. Highest frequency of interest is about 50kHz, but it turns out the people specifying this aren't really very clear what they mean by bandwidth, thus my low pass filter is set to 300kHz to give reasonably sharp risetimes for sharp-edged features.
Although Beta users found no EMC problems except ground loops with the predecessor product, I had trouble getting it through formal EMC testing due largely to poor rejection of radiated RFI pickup in the preamp, and conducted ripple getting in to the test system as a whole (several boxes joined by cables, but I'm not redesigning most of them) through the mains. Because the system was designed one box at a time, by different folk, the overall earth loops weren't properly thought out, but the weak point was the preamp I'm working on because small problems there were amplified. The emissions from the system were not, in general, a problem. So my personal target is to not just improve signal-to-noise ratio and bandwidth, but make the thing bulletproof to EMC. There are some constraints from what I have to interface to, but I have a lot of freedom as to how I implement the box I'm working on.
I had just this kind of problem with stuff I was designing for my fancy electron-microscope-based electron beam tester back around 1990.
We put together a subtracting (current-feedback) amplifier with a balun to ship a video signal around the machine without creating grounds loops. It seemed to work. I've got the circuit somewhere - e- mail me ( snipped-for-privacy@ieee.org will work) and I'll send you a scanned image of that part of the circuit,
We weren't too fussed about the DC accuracy at the time, but you'd have to find a currently available op amp for the job anyway.
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