AD515 Analog Devices, AD515 Datasheet - Page 4

• Manufacturer Part Number: AD515
• Description: Monolithic Precision/ Low Power FET-Input Electrometer Op Amp
• Manufacturer: Analog Devices

Typical Performance Curves
AD515A
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.
Figure 5. Open Loop Frequency Response
Figure 4. PSRR and CMRR vs. Frequency
–4–
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.
Figure 6. Input Common-Mode Range vs. Supply Voltage
Impedance and Bandwidth
Figure 7. Peak-to-Peak Input Noise Voltage vs. Source
REV. A