Digital communication over AWG18 cable

[m_t_c]

Hi all

There is a requirement to transfer 16-bit digitized signal (100-20KHz frequency range) over 500m long single wire (AWG18) with water acting as ground. What are the possible solutions for this?

Thanks & Regards

 

[FvM]

Can use 44.1 ksps (CD standard) or 48 ksps. Reliable and EMC aware transmission of 800 kbps data stream requires a modem.

 

[betwixt]

Are you suggesting you use only one wire to carry the data and the return path is water? That might be more of a problem over 500m. It would depend on how conductive the water is, whether interfering signals are present and how the single wire is positioned relative to (or submerged in) the water.

Brian.

 

[m_t_c]

Are you suggesting you use only one wire to carry the data and the return path is water? That might be more of a problem over 500m. It would depend on how conductive the water is, whether interfering signals are present and how the single wire is positioned relative to (or submerged in) the water.

Brian.

Yes, but there is a possibility to use two wires (each 18 AWG) also., Moreover, solution is needed to be very compact. Are there any commercial ICs available for this purpose?

 

[wwfeldman]

18 AWG is about 21 mohm/meter
500 m is about 1 ohm

a pair is about 2 ohms. so the wire is not a significant voltage drop
use twisted pair to reduce external noise pick up
or better, twisted, shielded pair like belden 8760
if it will be exposed to the weather, you should use something with a more weather resistant outer jacket

Texas Instruments differential line driver SN75158 and differetial line receiver SN75157 are both available from digikey
they are designed for RS422, with a max length of 1500 m

 

[betwixt]

Using two wires I agree with wwfeldman, RS422 is best. There are many devices you can use, MAX488/489 and ADM488/489 for example. the '488 versions have one transmitter and one receiver in an 8-pin package.
I seriously wouldn't consider using water as one of the signal paths.
Brian.

 

[m_t_c]

Texas Instruments differential line driver SN75158 and differetial line receiver SN75157 are both available from digikey
they are designed for RS422, with a max length of 1500 m

Thanks for help. If data is being digitized using 16-bit ADC operating at around 100 ksamples/s, would RS422 be able to handle this data rate over 500m cable or i need to reduce sampling rate?

Thanks & Regards

--- Updated Jul 18, 2020 ---

Using two wires I agree with wwfeldman, RS422 is best. There are many devices you can use, MAX488/489 and ADM488/489 for example. the '488 versions have one transmitter and one receiver in an 8-pin package.

Thanks for help. Currently i am using 16-bit ADC to digitize data at 100 ksamples/s. Will these ICs be able to communicate this data rate over 500m cable length?

Thanks & Regards

 

[KlausST]

Hi,

18 AWG is about 21 mohm/meter
500 m is about 1 ohm

Something is wrong here.
21mOhm/meter x 500meter = 10500mOhm = 10.5 Ohm

16-bit ADC operating at around 100 ksamples/s,

Incomplete information.
Mathematically this gives 1.6MBits/s. With UART includung Start abd Stop bits this means 2MBit/s
UART = NRZ, thus maximum (square wave) output frequency = 1MHz
But you need to add a frame sync signal to (at least) detect (at receiver side) which is High byte and which is Low byte.
..do you transmit a third byte?

Klaus

 

[m_t_c]

Hi,
Incomplete information.

Hi Klaus
What more information i needed to mention plz?

Mathematically this gives 1.6MBits/s. With UART includung Start abd Stop bits this means 2MBit/s
UART = NRZ, thus maximum (square wave) output frequency = 1MHz
But you need to add a frame sync signal to (at least) detect (at receiver side) which is High byte and which is Low byte.
..do you transmit a third byte?

I have not implemented any protocol yet. Currently, i am just searching the way to transfer ADC output over 500m cable (AWG 18 pair).

 

[KlausST]

Hi,

If you want us to do some bandwidth calculations, we need to know how you want to transmit the data.

RS422 is no protocol, it just defines the electrical signals.

More important than the AWG is a true characteristic impedance of your signal pair.

Klaus

 

[wwfeldman]

my apologies for dropping the decimal point

 

[m_t_c]

More important than the AWG is a true characteristic impedance of your signal pair.

The signal cable is Flexible 150 Strands 0.08mm of Tinned copper wire having impedance of 20m ohm/meter.

If you want us to do some bandwidth calculations, we need to know how you want to transmit the data.

Currently i have interface ADC with a FPGA to store data as well as to generate control signals. Now, how to proceed further for data transfer over 500m cable was my initial query.

 

[KlausST]

The signal cable is Flexible 150 Strands 0.08mm of Tinned copper wire having impedance of 20m ohm/meter.

This is somehow the opposite of what I was asking for.


Klaus

 

[betwixt]

The signal cable is Flexible 150 Strands 0.08mm of Tinned copper wire having impedance of 20m ohm/meter.

I rather think that is the resistance, not the impedance.
Although resistance plays a part in the calculations, it is the impedance that decides how the data is to be transmitted. The impedance should remain constant regardless of the length.

Brian.