# Is buffer needed for very long coaxial cable carrying DC analog signals coupled to an ADC with a huge input impedance

I know the topic is immensely broad but I dont have the practical knowledge on the following issue:

Let's say I have a very long like 1km BNC cable carrying DC signal around 1V analog voltage signal coming from a transducer. And let's say the BNC goes all the way straight to an ADC with 100 Meg-ohm input impedance at the end. If the measurement should be very accurate, do we need to buffer this signal?

BNC cable even 1km long has very very low resistance comparing to ADC's huge input impedance. Theoretically form this point I would think one wouldn't need a buffer.

But how would that be implemented in practice for 1V DC level and 10mV DC?

• Certainly I would first match the source(output)/load(input) to cable characteristics impedance. This has not so importance if you have a DC signal, but still...an low impedance buffer at source and amplifier at end is needed. Commented Jun 14, 2017 at 10:05
• There is no such cable BNC. BNC is the connector, while the cable is coax with its characteristics impedance as parameter. Commented Jun 14, 2017 at 11:11
• @MarkoBuršič Thanks for the correction I actually didn't know this and many of my friends ignorantly using this BNC term for the cable then. But if it is not too much for you could you add your answer how a DC signal got affected by in a 50 Ohm transmission line. How can I simulate this and see how DC is being affected? C and L are frequency dependent I cannot picture in my mind how DC voltage could be affected by L and C parameters of a coaxial cable. Commented Jun 14, 2017 at 15:27
• You didn't mention the transducer. How it is generating (sourcing) the signal, the 1km coax has pretty big capacitance, not sure if the transducer can drive the coax. The DAQ high input impedance and the long coax will act like an antenna collecting all the garbage signals from environment. You can try to mount the transducer on coax and you'll see what you will get. Commented Jun 14, 2017 at 16:59