15.3.2
10X Probes
10X probes (also called 10:1 probes, divider probes, or attenuating probes)
have a resistor and capacitor (in parallel) inserted into the probe.
Figure 15.14 shows the circuit diagram for the 10X probe connected to a high
-
impedance input of an instrument.
Rsrc
R1
R2
VS
VIN
C1
C2
Circuit under test
Probe
Input
Figure 15.14:
Input connection using a 10X probe
Assuming that R
src
is low compared to R
1
and that R
1
* C
1
= R
2
* C
2
, then the
effect of both capacitors cancel each other out in this circuit. The capacitor is
usually adjustable and can be tweaked for a nearly perfect match. In these
conditions, the relationship of VS to VIN is:
(EQ 4)
V
IN
=
V
S
R
2
R
1
+
R
2
R
2
is the input resistance of the instrument's high input impedance (1 MΩ) and
R
1
= 9 * R
2
. Using the previous equation, this results in:
(EQ 5)
V
IN
=
1
10
V
S
The final result is a probe / instrument input combination that has a much wider
bandwidth than the 1X probe due to the effective cancellation of the two
capacitors. However, the instrument now measures only one
-
tenth of the
original voltage (hence the name 10X probe). The circuit being measured is
affected with a load impedance of R
1
+ R
2
= 10 MΩ, which is much higher than
with the 1X probe.
GEN3i
362
I3763-3.1 en HBM: public
