National Instruments NI 4050 Manual De Usuario

© National Instruments Corporation

The NI 4050 has five input ranges available for measuring DC voltages. 
These ranges are ±20 mV, ±200 mV, ±2.0 V, ±25V, and ±250 V. Each 
range has a 10% overrange, except for the 250 V range. The 250 V and 
25 V input ranges have a 1 M

Ω 

input impedance; the 2 V, 200 mV, and 

20 mV ranges have an input impedance greater than 1 G

Ω

. Take these 

values into consideration when measuring high-impedance sources. When 
the NI 4050 is powered off, the 250 V and 25 V input range have a 1 M

Ω 

input impedance and the 2 V, 200 mV, and 20 mV ranges have an input 
impedance of 100 k

Ω.

If you are taking measurements that require a high degree of accuracy, you 
should consider problems associated with input impedance, noise effects, 
and thermal electromotive forces (thermal EMFs). These effects are 
discussed in the 

Figure 2-1 illustrates the input impedance of an NI 4050 and its effect on 
the measurement of a circuit under test. If you know the source impedance 
of the circuit being tested, you can correct for the attenuation caused by the 
NI 4050 in software. Since R

in

 is large, at least 1 M

Ω

, it will require a large 

source impedance, R

s

, to cause a large change in the measured voltage, V

m

.

Figure 2-1.  Effect of Input Impedance on Signal Measurements

Source
Voltage V

s

External Source
Impedance R

s

+

+

–

–

Input

Impedance

R

in

Measured

Voltage

V

m

V

s

R

in

R

s

+ R

in

V

m

----------------

-----------

=

HI

LO

Input

V

Ω