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A Simple Attenuator And
Preamp For DC Measurements
A battery powered attenuator
and preamp to extend the range of a voltage recorder.
Gains cover ±40db, ±20db, and 0 db
and can run for thousands of
hours on a single coin cell.
To keep things simple, a terminal block is used to connect
(from left to right) INPUT, COMMON, and
After building the
voltage recorder with playback, the first use I had
planned for it was to record the activity
of the water pump that supplies my household needs. This was going to
require detection of signals in the tens of
millivolts. The philosophy behind the voltage recorder
was to make is very straight-forward
and as simple as possible, and to
make adapters as necessary for
particular measurement applications. It was clear that
I could take care of many
of these applications by making an
attenuator and preamp.
I had a few New Japan
Radio Co. LTD NJU7051 single
opamps that I had
bought years ago. They have
adequate offset voltage,
offset input bias current, and input bias
current specifications and
-this is the good part -are rail-to-rail
that can operate with
a single power
supply as low as 1 volt drawing
less than 20 uA.
This particular part may
be obsolete now,
but I fond
from New Japan
is so low
when driving high
an off position
in the rotary
needed or not,
its like the OFF
button on a
gives the use
peace of mind.
The gains are -40 db, -20 db, 0 db, +20 db, and +40 db, which correspond to gains of X
1/100, X 1/10, X
1, X10, and X100 respectively.
There is a
single common ("GROUND")
connection shared by
input and output signals.
In the -40, -20,
and 0 db modes, the opamp is connected as a high
input resistance voltage
follower. The attenuation is accomplished with resistors, with the input
resistance being between 1.1 Meg and 1.11 Meg Ohms.
In the +20
and +40 db modes, the opamp is configured
as a non-inverting amplifier.
The input attenuator is made with a 1 Meg
Ohm input resistor in
series with the input signal, which is followed by
a 10k resistor in the case of 40 db
attenuation, a 111k resistor in the case of 20 db
attenuation, and no resistor in the case of 0 db attenuation. To reduce stress
on the opamp's
inputs in the case of excessive voltage being
applied to the input, two diode-connected JFETs
are used in conjunction with the
1 Meg input resistor to limit current.
One JFET clamps the incoming signal to the positive power supply and the other JFET
clamps the signal to common (or "GROUND").
I started with Schottky diodes, hoping I would get lucky and get reverse leakage
current a low lower than the
but no such luck. The resulting
offset at the input was comparatively huge at 15 millivolts at room
temperature, which would
double every 10 °C.
The 2N5485 JFETs have a very
low reverse gate current of
only 1 nA.
easily live with
in case the
is exposed to
a high voltage
battery is not
Connected at the
same point in
the circuit as the protection
diodes, is a 0.022 uf capacitor which is
intended to limit bandwidth to reduce
noise some. The
driven by a low resistance
to be (I did not
0 db, +20
db, +40 db 7.2 Hz
A slightly regrettable effect of having only a two pole rotary switch avalable
for mode selection is that
the resistances seen by the inverting and
non-inverting inputs are quite different from each
other in the -40
and +40 db modes. The resulting difference results in a small DC offset at the output.
The opamp, the 1 Meg Ohm feedback resistor,
the battery holder, the Zener diode and bypass capacitor are
mounted on a hand-made circuit board made of a predrilled
prototyping board with one pad per hole on 2.54 mm centers.
Mounting the SO packaged opamp took a little time, but it
was done successfully not once, but twice because I had
destroyed the first opamp and had to replace it. I found
that I could accommodate two IC leads on one pad by cutting
the pad in half.
No printed circuit
board was needed, but it would have made it nicer to