AD515A

–4–

REV. A

COAXIAL CABLE AND CAPACITANCE EFFECTS

If it is not possible to attach the AD515A virtually on top of the

signal source, considerable care should be exercised in designing

the connecting lines carrying the high impedance signal. Shielded

coaxial cable must be used for noise reduction, but use of

coaxial cables for high impedance work can add problems from

cable leakage, noise and capacitance. Only the best polyethylene

or virgin teflon (not reconstituted) should be used to obtain the

highest possible insulation resistance.

Cable systems should be made as rigid and vibration free as

possible since cable movement can cause noise signals of three

types, all significant in high impedance systems. Frictional

movement of the shield over the insulation material generates a

charge that is sensed by the signal line as a noise voltage. Low

noise cable with graphite lubricant such as Amphenol 21-537

will reduce the noise, but short rigid lines are better. Cable

movements will also make small changes in the internal cable

capacitance and capacitance to other objects. Since the total

charge on these capacitances cannot be instantly changed, a

noise voltage results, as predicted from:

∆V = Q/∆C. Noise

voltage is also generated by the motion of a conductor in a

magnetic field.

The conductor-to-shield capacitance of coaxial cable is usually

about 30 pF/foot. Charging this capacitance can cause consider-

able stretching of high impedance signal rise time, thus cancel-

ling the low input capacitance feature of the AD515A. There are

two ways to circumvent this problem. For inverting signals or

low level current measurements, the signal is carried on the line

connected to the inverting input and shielded (guarded) by the

ground line as shown in Figure 2. Since the signal is always at

virtual ground, no voltage change is required and no capaci-

tances are charged. In many circumstances, this will destabilize

the circuit; if so, capacitance from output to inverting input will

stabilize the circuit.

Noninverting and buffer situations are more critical since the

signal line voltage and therefore charge will change, causing

signal delay. This effect can be considerably reduced by

connecting the cable shield to a guard potential instead of

ground, an option shown in Figure 3. Since such a connection

results in positive feedback to the input, the circuit may be

destabilized and oscillate. If so, capacitance from positive input

to ground must be added to make the net capacitance at Pin 3

positive. This technique can considerably reduce the effective

capacitance that must be charged.

Typical Performance Curves

Figure 4. PSRR and CMRR vs. Frequency

Figure 5. Open Loop Frequency Response

Figure 6. Input Common-Mode Range vs. Supply Voltage

Figure 7. Peak-to-Peak Input Noise Voltage vs. Source

Impedance and Bandwidth