Keithley KPCI-3110, User Manual

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KPCI-3110 and KPCI-3116 User’s Manual Using Your Own Screw Terminal Panel D-3
Differential inputs
Differential inputs offer the maximum noise rejection at the expense of half your total channel
count. For the best results, shielded twisted pairs are a must. The shield must connect at one end
so that ground currents do not travel over the shield. In low-level voltage applications, differen-
tial inputs reduce problems not only due to electrostatic and magnetic noise, but due to cross-
talk and thermal errors.
One problem to consider with differential inputs is the bias current error. The differential imped-
ance is usually hundreds of megaohms. With a very small bias current multiplied by this high
input impedance, the voltage produced is out of the common-mode input range of the instrumen-
tation amplifier.
An external resistor must be provided to return this bias current to the analog common of the
data acquisition board. This resistor is typically in the order of 1k
Ω
to 100k
Ω
from the input low
side to analog common. Alternatively, the external common can be returned through a 10
Ω
to
100k
Ω
resistor to analog common (it cannot be
Ω
due to ground currents).
Analog outputs
Most data acquisition boards have a minimum of two analog output channels, with a resolution
of 12 to 16 bits (even though the accuracy may be less).
On all Keithley boards, we ensure that the analog outputs do not break into a high frequency
oscillation with high capacitance loads that may be experienced with long cables. Typically, the
analog outputs drive 1,000pF without degradation and bandwidth-limit with higher capacitive
loads.
The grounds of most boards are optimized for analog inputs at the expense of some logic or
high-frequency noise on the analog outputs. This is because the analog and digital grounds of
the board are connected at the ADC’s input.
The analog outputs are brought out as a high and a low signal, but the low side is the analog
ground at the DAC’s output buffer. To remove the high-frequency noise and smooth the glitch
energy on the analog outputs, you can install a 15kHz RC filter on the output, a 100
Ω
resistor in
series with the output, and a 0.1
µ
F capacitor between the output side of the 100
Ω
resistor and
output low.
Digital inputs and counter/timer inputs
TTL-type inputs must have current limiting so that circuitry is not damaged when power is
removed. On all Keithley PCI boards, current limiting is used to prevent damage in this fault
condition.
On high-speed clock inputs, a ground that is located in the connector next to the clock must be
connected as a twisted pair with the high-speed clock input.