201303.pdf 36567KB Mar 22 2013 09:11:22 PM

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Circuit maximizes pulse-widthmodulated
DAC throughput
Ajoy Raman, Bangalore, India
↘
Simple DACs realized by lowpass
filtering microcontrollergenerated
pulse-width-modulated
(PWM) signals have a response that is
typically a tenth of the PWM frequency.
This Design Idea is a novel implementation
of a previously published
method 1 employing a reference ramp
whose output is sampled and held by
the PWM signal. This approach results
in a throughput rate equal to the PWM
frequency.
You can use the circuit in Figure 1 to
implement a ±10V 10-bit DAC with a
throughput of 20 kHz. A DSPIC30F4011
microcontroller (not shown) is operated
at a clock frequency of 96 MHz to generate
the capture signals OC 1
and OC 4
.
Clock/4 is fed to an internal 16-bit timer
whose period is set for a count of 1200
corresponding to a PWM frequency of 20
kHz. Signal OC 4
is mostly high and goes
low at a fixed count of 1170 as a reference
for ramp generation. IC 1A
, along with
Q 1
, forms a precision constant-current
source that linearly charges capacitor C 2
when Q 2
is off. This signal inverted by
IC 3A
switches Q 2
on for a period of 30
counts to discharge C 2
for the start of the
next ramp. IC 1B
buffers, amplifies, and
offsets the ramp; potentiometers R 2
and
R 5
adjust the offset and gain.
The OC 1
falling edge controls the
PWM DAC sample timing relative to
DIs Inside
52 A circuit for mains synchronization
has two separate
outputs for each half-period
55 Low-component-count
zero-crossing detector is
low power
57 DC-DC converter starts up
and operates from a single
photocell
58 Filter quashes 60-Hz
interference
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the ramp voltage. The data word to be
converted determines the OC 1
duty
cycle by comparing it internally in the
15V
15V
5V
R 1
2.7k
R 2
1k
R 3
18k
–15V
R 4
56k
PIN 19
OC 4
1
OUTPUTS
FROM
DSPIC30F4011
14
5V
7
C 3
680 pF
R 9
1.8k
R 5
10k
R 7
18k
IC 3A
CD40106
5V
2 4
−
IC 1A
LF353
3
+
8
15V
R 8
2.2k
IC 3B
CD40106
PIN 23
3
4
OC 1
2
1
Q 1
BC177
Q 2
BC107
R 6
1k
C 2
0.1 μF
MYLAR
6
−
IC 1B
LF353
5
+
7
ANALOG IN
S/H IN
15V
3
1 2
IC 2
LF398
8
4 7 6
–15V
5
C 1
1800 pF
DAC
OUTPUT
Figure 1 The off-page microcontroller generates signals for ramp control (OC 4
) and sample timing (OC 1
).
[ www.edn.com ] March 2013 | EDN 51