# Input impedance, Dither, Digitizing the signal—the adc – National Instruments NI PXI-562X User Manual

## Page 17: Input impedance -3 dither -3, Digitizing the signal—the adc -3, Conditioning the signal—impedance, dither, Gain, and ac coupling

*Chapter 2*

*Hardware Overview*

© *National Instruments Corporation*

*2-3*

*NI PXI-562x User Manual*

**Conditioning the Signal—Impedance, Dither, Gain, and AC Coupling**

To minimize distortion, signals receive a minimal amount of conditioning.

Gain and coupling are nonadjustable. The NI PXI-5620 is AC coupled,

meaning it rejects any DC signal components. The NI PXI-5621 is DC

coupled, meaning its wider passband acquires DC signal components also.

Both versions of the NI 562*x *digitizer module have a set input impedance

of 50

Ω and may apply dither to the input signal.

**Input Impedance**

The input impedance of the NI 562*x *and the output impedance of the source

connected to the NI 562*x *form an impedance divider, which attenuates the

input signal according to the following formula:

where *V*

*m*

is the measured voltage

*V*

*s*

is the unloaded source voltage

*R*

*in*

is the input impedance of the NI 562*x*

*R*

*s*

is the output impedance of the external device

If the signal you are measuring has an output impedance other than 50

Ω,

your measurements are affected by this impedance divider. For example,

if the device has 75

Ω output impedance, your measured signal has 80%

of the voltage it would have at 50

Ω.

**Dither**

Dither is random noise added to the input signal between 0 and 5 MHz.

Dither lowers the amount of distortion caused by differential nonlinearity

in the ADC when a signal is digitized. When an FFT is applied to the signal,

this random noise cancels out most of the distortion created by differential

nonlinearity. Dither is not automatically applied, but you can enable it in

software.

**Digitizing the Signal—The ADC**

Regardless of your requested sample rate, the NI 562*x *ADC is always

running at 64 MS/s. If you request a rate less than 64 MS/s, the timing

engine of the NI 562*x *stores only one sample in a group of *n *samples,

effectively reducing the sample rate to 64/*n *MS/s.

*V*

*m*

*V*

*s*

*R*

*in*

*R*

*in*

*R*

*s*

+

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