With a decade box that goes to 999,990
Ω
, and a 4.7K
Ω
resistor for the
shunt, we can get a wide range of amplification, as shown in the following
table.
Arithmetic Gain
R1 (Shunt)
R2 (Feedback)
1
4700
Ω
0
1.212
4700
Ω
1000
3.12
4700
Ω
10,000
22.2
4700
Ω
100,000
213
4700
Ω
999,999
Once we have found the right resistor combination, we can remove the
decade box, replacing it with a single resistor.
One thing to note with the non-inverting amplifier is that it can only increase
the input, not reduce it (there’s no way to select resistors that get rid of the
1
in the equation). The inverting amplifier also uses two resistors to
configure gain, but can be configured for fractional (less than 1) gain,
though this comes at the cost of other complexity. As the name states, this
configuration inverts the signal at it’s output - a situation that can be solved
in software, but is also somewhat counter-intuitive.
Resources and Going Further
Decade boxes are handy to have on your workbench - you never know
when you’ll need to dial in a resistance.
Resources
• Banana to IC Hook or Banana to alligator leads are useful for
connecting the decade resistance to other circuits.
• Building a non-inverting amplifier? You’ll need some operational
amplifiers.
• If you need small resistances that can dissipate more power, check
out our Power Resistor Kit, rated to 10W each.
Going Further
• Theory behind the Wheatstone Bridge.
• Moving beyond the Wheatstone bridge, the Wein Bridge uses similar
nulling techniques to measure capacitors, and a Maxwell Bridge
measures inductors.
• An interesting anecdote from Howard Johnson regarding where
precision resistors come from.
Page 16 of 16
8/5/2016
https://learn.sparkfun.com/tutorials/decade-resistance-box-hookup-guide?_ga=1.111609284....
