Sensor cable design

Ground, or Vcc, as you prefer. Then again, two coax with one carrying Vcc on the inner and the other carrying signal on the inner, or one coax and another wire for Vcc might be preferable.

Your frequency is in the HF range, so you should seriously consider impedance matching to the cable.

If you want to use twisted pair, the signal must be symmetrical to achieve advantage of the cable. For single-ended signals, consider coaxial cable instead.

Remember that a RF signal travels in the insulator between the conductors.

Ecnerwal, thanks for your reply.

Would twisted triad be better than two twisted pairs? If I used two twisted pairs with ground on one wire of each pair, which ground wire would carry the return current? Or would the return current be split between the two ground wires?

Doug

Tauno, thanks for your reply.

I was going to use a 100 ohm series resistor at the output of the op-amp. Is this the proper way to match the cable impedance?

I was hoping to use twisted pair since it is much cheaper than coaxial cabl e. If I bring back the signal and ground wires in the same twisted pair, w ould that be considered symmetrical? More current would be flowing out to the sensor over the Vcc wire than would be returning through either the sig nal or the ground wires.

Thanks, Doug

John, thanks very much for your reply.

I'm still a bit confused about single-ended signals. In this application, the sensor is floating--it's not connected to earth ground. If the signal is referenced to the sensor's ground wire, and I return the ground wire alo ng with the signal wire, wouldn't those two wires be balanced?

I've also been confused by how the return currents would flow. Would they flow in both twinaxes, with the currents equal and opposite in each twisted pair? How do the ground currents know which return wire to take?

What's the signal level? What sort of s/n ratio do you need?

Twisting single-ended signals probably doesn't matter.

One less-obvious EMI problem is RF (like from TV and radio stations, cell phones, motor controllers, things like that) getting rectified by opamp front ends... or back ends. That will be site dependant. Shielding would help with that.

The cable is 2 meters long. The signal wavelength is 60 meters. There will be no significant transmission-line effects.

The cable can be treated as a lumped capacitance, probably under 200 pF, and most opamps won't mind that load. But it never hurts to check, and a series resistor might be a good idea.

If the sensor signal is truly AC, you could use a coax cable and send the power down the coax along with the signal.

Besides the in-circuit properties, endedness refers to the field given off by the cable (or, equivalently, how the wires see the field around). If the construction is symmetrical (a plus and a minus wire, same size, evenly spaced, etc.), then the signals should be symmetrical as well. If not, an average field radiates (or an external field will be picked up, same thing).

Return current follows whatever wires are close to the signal in question. If other signals are nearby, they act as grounds, too. Imagine ribbon cable: if you put a bunch of signals in a row with one ground at the end, all those signals are going to screw with each other. If you alternate ground, signal, ground, signal.., it's so much better (like 20dB). There's an appnote somewhere out there with scope pictures illustrating the difference.

Note that supply and ground are, for the purposes of signal quality, both grounds. This is especially true with bypasses at *both* ends.

Tim

-- Deep Friar: a very philosophical monk. Website:

I'm still a bit confused about single-ended signals. In this application, the sensor is floating--it's not connected to earth ground. If the signal is referenced to the sensor's ground wire, and I return the ground wire along with the signal wire, wouldn't those two wires be balanced?

I've also been confused by how the return currents would flow. Would they flow in both twinaxes, with the currents equal and opposite in each twisted pair? How do the ground currents know which return wire to take?

--- The relationship between the sensor's output and the input to the opamp is immaterial, according to your original post:

"I'm having difficulty finding a suitable cable for a wired sensor application, and I was wondering if anyone could check my design approach. Basically, my control circuit powers a wired sensor located about 2 meters away. The sensor circuit uses a single-ended op-amp to amplify a 5 MHz transducer signal, and then sends it back to the control circuit where it is digitized. So, I need 3 wires between the control circuit and the sensor: Vcc, ground, and signal."

Since the output of the opamp - not the sensor - is what's being sent to the control circuit, is single-ended, and is referenced to ground, then it isn't a balanced signal.

---

--- "Twinax" is, nominally, coaxial cable with two insulated inner conductors twisted around each other and surrounded by a common shield, and you fix it so that the ground currents have no choice but to return via the shield or the shield's drain wire if it's a foil shield.

To use it in your application you'd send Vcc from the control circuit over one conductor to the sensor package, the 5MHz signal from the opamp over the other conductor, and you'd connect the power and signal grounds to the shield.

Personally, I'd use a piece of regular coax to get the signal from the opamp to the control circuit, a piece of hookup wire to get Vcc from the control circuit to the sensor package, and the coax's shield as the common ground for them both.

You could even lazy-wind the Vcc wire around the coax's jacket just to make it look purty.

-- JF

John, thank you very much for explaining this to me. After 25 years, I fin ally understand the difference between balanced and unbalanced cables. "Ba lanced" means balanced _current_ flow on two wires going in both directions . That's why coax is used for unbalanced cables--the shield stays at groun d potential while the signal _voltage_ varies in the center conductor.

As John Larkin pointed out above, the cable is physically short compared to the signal's electrical wavelength, so transmission-line reflections will have time to dissipate and won't matter at my relatively slow signal freque ncy. So, I don't have to consider impedance matching, other than for the o p-amp output. This means I don't need expensive coax with its controlled i mpedance--simple foil-shielded, unbalanced wire should work to protect agai nst cross-talk and external EMI.

Thanks again! Doug

John, thank you very much!

"Doug D."

I'm having difficulty finding a suitable cable for a wired sensor application, and I was wondering if anyone could check my design approach. Basically, my control circuit powers a wired sensor located about 2 meters away. The sensor circuit uses a single-ended op-amp to amplify a 5 MHz transducer signal, and then sends it back to the control circuit where it is digitized. So, I need 3 wires between the control circuit and the sensor: Vcc, ground, and signal.

The technique is known as "phantom powering".

Small ( ie 100uH ) inductors at each end of the +5V line will easily allow a 5MHz signal to be superimposed on the same wire.

BTW: "tm" has already alluded to this idea and JF posted a schem.

Twisted pair should work as well as co-ax for your app - long as no nasty RF exists in the vicinity.

... Phil

ble. If I bring back the signal and ground wires in the same twisted pair, would that be considered symmetrical? More current would be flowing out t o the sensor over the Vcc wire than would be returning through either the s ignal or the ground wires.

How cheap did you want it? You can't beat Ethernet Cat 5 SFTP patch cables for cheap, in addition to maximal EMI performance both from external source s and between pairs, and reduction of radiation from the cable signals. Thi s one is $6.99 from Amazon, already connectorized:

Your signal may be 5MHz ( is that total BW?) but the potential interference may not be.

Practically all cables have equal current flow in opposite directions - even AC power cable.

Balanced refers to the conditions at each end of the cable.

1. There is equal and opposite current flow in the inner and outer conductors.
2. There is no external magnetic or electric field.
3. External electric and magnetic fields do not affect the cable.
4. Single ended drive and receive is used.

However, a twisted pair cable can do virtually the same - but only if it is ES shielded.

If the send and receive signals are both balanced, then you can lose the ES shield in most applications.

... Phil

The impedance of a twisted pair cable is quite close, CAT5 is around 120 ohm.

No. You need to have +signal and -signal to prevent asymmetric ground currents. The DC polarities do not need be opposite, but AC components need.

I assume you have both the signal gnd (common) and the power gnd (common) connected at your controller? It is NEVER a good idea to use single ended to transfer energy around, should have used balanced receiver at the controller and balanced driver at the sensor. However, you can probably get by with your architecture.

Two tiwsted pair should be the cheapest. You can use uncontrolled like telco phone line, or controlled cat 5 type cabling, your choice.

one pair - Vcc and GND

at the source, your OpAmp, place a common mode choke in series with the the signal/gnd wires to make them 'float' relatively.

Why? because you actually have an 'extra' connection to Earth that you have NOT taken into account - the parasitic capacitance of your sensor. Assuming you put some type of shielding all around it, which is referenced to its ground, that in combination with the unbalanced currents in your cabling can lead to havoc everywhere, with NO easy solutions. You mentioned EMI coming IN. Don't forget about EMI going OUT!

But at least with the above, you might squeak by, and worse case make that twisted pair inside shield should be ok for sure.