62 Silicon chip Australia’s electronics magazine siliconchip.com.au
Now let’s look at how it’s used in
the lowest cost module of our three.The ML005 module
Fig.2 shows the full circuit of the
ML005 module, plus the basic map of
its PCB. As you can see, this module is
essentially a ‘bare minimum’ design.
It contains little more than the AD584
chip plus a few support components
and some SIL headers used for input
and output connectors, and for pro-
gramming the desired output voltage.
It uses the “JH” version of the AD584
chip, so we shouldn’t expect too much
from it in terms of output precision or
temperature stability.
Diode D1 is presumably to protect
the AD584 from damage from reversed
supply polarity, while LED1 and its
rather high-value series resistor is to
provide power-on indication.
The 10nF capacitor connected be-
tween pins 7 and 6 of the device re-
duces the output noise level, whilebe used to source 10V, 5V or 2.5V inde-
pendently, although pins 2 and 3 can-
not provide significant current without
affecting accuracy and so if used, the
voltages should be fed through unity
gain buffers. More on that later.
Note that you can’t get a buffered
1.25V output from pin 1 by tying pins
1 & 6 together, turning the op amp into
a unity gain buffer. This is because the
2.5V tap is used for internal biasing.
There are two pins we have not yet
explained in Fig.1: pin 7 (CAP); and
pin 5 (STROBE). Pin 7 is provided
so you can connect a small capacitor
(usually 10nF) between this pin and
pin 6 (Vbg), to lower the bandwidth
of the internal op amp and reduce the
output noise level.
Pin 5 is provided to allow the AD584
to be switched on or off by a logic sig-
nal. If no current is drawn from pin
5, the device operates normally, but
if the pin is pulled down to common/
ground, it effectively switches off.Fig.1. At the heart of the device is a
high stability band-gap reference di-
ode providing a 1.215V reference. This
is followed by an op amp used as a
buffer amplifier, with its voltage gain
set by the string of divider resistors
connected between its output (pin 1)
and common (pin 4) terminals.
Internal feedback from the lowest
tap of the divider string (pin 6, Vbg)
ensures that the buffer amp maintains
Vbg at very close to 1.215V, the band-
gap voltage. So if a DC voltage between
+12-15V is applied to the device be-
tween pins 8 and 4, and no external
connections are made to pins 2, 3 or
6, it will provide a nominal output
voltage of very close to 10V at pin 1.
But if pins 1 and 2 are joined exter-
nally, the voltage at pin 1 will drop to
very close to 5V, and if pins 1 and 3
are joined, it will be very close to 2.5V.
If pins 2 and 3 are joined, it will settle
very close to 7.5V.
Notice also that pins 1, 2 and 3 can
Fig.2: the circuit and general layout of the basic ML005 reference board. It’s a minimalist implementation of an AD584-
based voltage reference, with pin header J5 provided to select the output voltage using a jumper shunt.
Fig.1 (left): the AD584 voltage
reference IC used in all these
modules contains a very accurate
and stable 1.215V laser-trimmed
bandgap reference, plus a precision
op amp and resistors to amplify
that reference to provide four
possible output voltages (2.5V, 5V,
7.5V & 10V) depending on which
combination of pins 1, 2 & 3 are tied
together.Right: the ML005-V1.2 module
shown at nearly twice actual size.
Note that searching for “ML005”
online will not find this module, so
you will need to search for AD584.