08/25/94
15:00
DGM-1040 and -1080
OfA Deg litchers
--. COMPUTER LABS
..'!
~ 002/005
ANALOG DEVICES
11'9106680101
LIllI ANALOG DEVICES
General
Deglitchers are normally used
to eliminate the non-linear effects
of "glitches" from the output of
D/A converters. The Computer
Labs DGM Series of Deglitchers
may be used with almost any type
of D/A converter to generate an output signal with extremely high
spectral purity. These Deglitchers
have been used for television signal.
reproduction, CRT displays. waveform generation, and automatic test
equipment, to name a few.
I
The Problem
I
OBS
I
OLE
In most instances, fast-settling
DIA converters are current-switching types, rather than voltage types.
In this type of converter, any input
bit changing causes a change in the
output current of the converter.
The input circuits for currentswitching 01A's, and their TTL or
DTL driving logic, are subject to
the characteristic of saturated logic
which causes propogation delay for
negative-going inputs to be different from the delay for positivegoing inputs.
As a result of this phenomenon,
time skew of the individual current
switches within the D/A converter
is worst when one or more input
bits are out of phase with th e others.
This is true even for ideal inputs in
which the digital input bits arrive
simultaneously; if there is time
skew among the bit inputs, the
problem becomes even more pronounced.
These differences among the
internal switches cause a discontinuity or "glitch" in the output
current of the D/A converter. The
true "worst case" glitch always
occurs at the switching point of the
Most Significant Bit (MS8) or the
center point of the output range,
because nearly equal and opposite
currents are being switched.
In addition to this inescapable
switching transient in the output,
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15 nsee acquisition
0.01% linearity.
time.
Directly interfaces with ultra high-speed, current-output
TTL or ECl compatible.
DGM plus D/A costs less than any "degfitched"
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D/A's.
O/A available,
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15:01
most current-output D/A converters
also have a maximum output voltage
AC CHARACTERISTICS
limitation.
Part of this limitation may be
the result of high output capacitance and/or resistance; this characteristic can be "masked" in data
Droop Rate
Harmonic Distortion
Feedthru Rejection
Pedestal
Residual Glitch
Output Noise level
OBS
NOTE;.: Addlt;o~al d.f.il. on 'ha cnar8ct~(i~'iC5 cf current,wltchi~!I. r85I...nli~g
O/A Con'~r!."
ore i~cl1Jded i~
"Nol.. on F""$ettll~g
D/A Co~.erters'.
which 8'. pan 01
Ih. oale Sneel on Ihe Compu'er lab. MDSfMDP $e,les
O/A'S.
The Solution
When first exposed to the undesirable
effect
of analog
discontinuities
in the outputs of current-switching
D/A
converters,
many users erroneously
assume
two straight-forward
approaches
will solve the problem. At the input, minimizing
time skew among
the data bit inputs will be a use-
ful technique.
At the
output.
the
use of a filter may also appear to be
a neat "solution" to the problem.
Unfortunately.
no amount of
time alignment
on the input bits
will overcome
the physical laws
associated
with the propogation
delays of saturated logic discussed
earlier. In addition, a filter designed
for eliminating
the "glitch" at the
major
carry
point
will not be
optimized
for transients
at other
points of the output,
nor will it
change the relationship of transient
amplitudes;
this is because the
glitch is a function of signal dynamics. AS a result, a multitude of
intermodulation
products are formed; some of these 1M products appear in the video pass-band as
spurious
signals,
and increased
noise
level.
Besides these considerations,
no amount of minimizing time skew
DGM-1040
DGM-10aO
15 nsec
30 MHz
10 nsec
75 nsec
11 MHz
10 nsec
6 nsec
Acquisition Time
Sample Rate (max)
Sample Delay TTL
ECl
sheets if settling time is specified
with an impractically-low
impedance load.
Even D/A converters
which
have low output capacitance and/
or resistance will have a maximum
output voltage limitation which is
established
by saturation
of the
internal switching transistors.
This internal
saturation,
in
turn, generally precludes operating
the current-switching
D/A converter as effectively in a unipolar
mode as it can be operated in a bipolar output mode.
I4I003/0~
ANALOG DEVICES
'a'9106680101
6 nsec
.
,
8 mY/usee
>60 dB
>60 dB
10 mV
30 mV
0.2 mV (AMS)
See Fig, 2&3
s~ Fig. 2
mY/usee
>60 dB
>70 dB
2 mV
$"" Fig. 1
20 mV
S""FiQ.1
0.1 mV (RMS)
DGM-1040 & DGM"1080
DC CHARACTERISTICS
Gain
Offset
OLE
Offset Drift
Linearity
Current Source Current
Current Sink Current
.975
Adjustable to zero
100 ppm/oC
:1:0.01%
7.5 mA max
19 mA max
TE
ANALOG INPUT CHARACTERISTICS
Input Voltage
Input Impedance
Input Bias Current
i:2 V
1 megohm
0.05 nA
ANALOG OUTPUT CHARACTERISTICS
Output Voltage (no load)
Output Current
Output Impedance
:1:2V
:!:SO mA
50 ohms
DEGLITCH STROBE CHARACTER ISTICS
Optionally either of the following:
TTL single line input
"0" = track
"1" =hold
ECL 2 line complementary
"0" = track
"1" = hold
0 to +0.4 V
2standard
+2.4 to +4 V
TTlload!
-1.7 V
-0.8 V
POWER REQUIREMENTS
+15 V @ 100 mA w/o current source connected
-15 V @ 100mA w/o current source connected
+5 V @ 20 mA
TTl input option
-5.2 V @ 80 mA
-5.2 V @ 24 mA
}
ECl
input option.
PHYSICAL CHARACTERISTICS
Package Size
2.3" by 2.3" by 0.43"
58 mm by 58 mm by 11 mm
Weight
Pins
Case
3 oz
~5 grams
0.040 diameter gold plated
diallyl phthalate per Mll-M-14
type SDG-F
The.e inpu,,"a'e
".ch terminated
wit"
a 330 ohm
pull-
down ,a~l~tor to
.5.2 v.
~
15:01
08/25/94
DGM
OUTPUT
DC
f
WITH
INPUT?
~UA"
-1- MOLD
.-. .,
I
OEGLITCH
INPUT "0" TRACt(
1
--,----
-
CATA CHANGING DATA
ST~ADT , GLITCH PERIOD
~
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DEGL.ITer
J
INPUT:L
~
-"-
1
FINAL SECOND VdLUE
OBS
ANAL.ex;
OUTI"IJT
OF DIA
INITIAL.
VAL.uE
ANI.LOG
OUT OF
DGM
1
--J L
INITIAL
VAL.UE
SAMPLE
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ACOUISTION
TIIoIE
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DE:L.oI.Y
FIG,2
OUTPUT - D/A
DGM
0.'
GLITCH
FIG, I
OUTPUT
DC INPUT
DGM
OIl.
INPuT
and/or
filtering the output will
overcome the output voltage limitations imposed by internal switching transistor saturation.
An optimum solution to the
problem of glitches would cause
the glitch to remain constant, regardless of the transition
points
on the input data. As an example, it
should remain the same for the
transition
from a 000 000 001 to
1 000 000 000; as it is for the
transition
trom 1 000 000 000 to
1 000 000 001; or any other two
input words.
Ideally, an optimum solution
to the glitch problem would also
permit using the full current drive
capabilities ot the current-switching D/A converter in both bipolar
and unipolar modes ot operation.
The purpose of the Computer
Labs DGM modules is to reduce the
amplitude of the glitch to an acceptable level, and more importantly, to
provide a constant-amplitude
glitch.
~OOP
PEDESTAL.
1"/. OF fiNAL. VAL.uE
INPUT
This
curve
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100
.COUISIT,ON
TINt
FIG,
ACQUISITION
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Sli:TTUNG ACCURACy
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fine line, i.e..
at the sample
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a single-line spectrum
rate frequencies. and
harmonics of the sample frequency.
The deglitcher circuits effect.
Ivery eliminate the intermodulation
products discussed earlier. When
they do. the SIN ratio approaches
that of an ideally-quantized signal.
where the rms noise is q/ v'l2.
when frequencies above Nyquist
are filtered out.
The DGM modules also incorporate an internal adjustable
current sink. to adjust the D/A output
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will hold the area under the
at a constant
value; the
modules are not intended to get
rid of all glitches per se.
When the area under the transient curve is held constant, the frequency spectrum of the glitch is a
AT 7~ .s
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It
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141 004/005
ANALOG DEVICES
'5"9106680101
for
dition,
bipolar
operation.
an internal
voltage
In adshifter
consisting of a current source and
a precision
resistor
allows
using
the bipolar output drive capabilities
of the DfA converter for unipolar
positive
applications.
These
fea-
tures permit OV to +2V unipolar
outputs. An internal buffer amplifier
buffers output
loading from the
D/A module. permitting a constant
SO.ohm output impedance.
Non,:
Ado."onoi
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15:02
08/25/94
'a'9106680101
~ 005/005
ANALOG DEVICES
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BIPOLAR
ANALOG
OUTPUT
SO Okllll 0.1 V no-PI
LINE
13.I:I,COW"ON
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OUTPUT
14
21
14
11
MDS
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....
(I)
...
....
CIA
- OElIS
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DGM
SO A
-
1040
22
v 0 IOP~[tTi
OEGLITCH
CONvERTER
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MODULE
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V (P-PI
Co :501\.
CLOCK
..'
-1
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SETT\,ING TII.'E: . 20 NS
"'.!loX,UPDATE RATE - 30 MHl
DISTORTION < 0,1 '/,
L.!IoIiGE SIGNAL BANOWIDTHS
UP TO 15MI1Z
L
1:1 1'5
(MINI
-IS
I--
V
ULTRA FAST OEGLITCHED CIA CONFIGURATION
OLE
REF'N
+v
,REFOUT
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,-",
11
1000
OBS
STROBE
IS
OFFSET
ADJUST
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<liS
t.4DSL-0825
ai:>
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-0
MDSL-
...
50 jl
1035
Gl:N
<;>::
IOUT
'C;
~Q
...
...
0
17
,
DEGLITCH
MODUI..E
OFFSET
.!IoDJUST
1000
A
-.J
L
t
45 /oIS
(MINI
DATA UPOATE
I.£ADING
OUTPUT: 0 TO +I VOLT INTO 1S JI.
SETTLING TIME'
20 NS
MAJ(. SAMPLE: !lA-TE' II WHZ
DIFF, GAIN ~4"1,
'881T51
~ 2 ":t. (9 BIT51
2 I % 110 BITS)
DII'I'. PHASE: 1: 2 eEG. <s BITS!
~ I DEG. (9 BITSI
~ 0.5 DEG. (10 BITS)
11
-v
I-SYNCHRONOUS
EDGE: OF STROBE
7SJI.
OGM-1040
1600AI
<p
~S J
STROBE
24 J>.
20
!IV
ANA\,OG
SIGNAL
CIA CONvERTER
~
O'rO+IV
15 OHIO! VUIE:O
LINE
SIGNAL
ZI
OR
0
TE
wITH
DEGLITCHEO CIA FO~
TELEVISION SIGNAL REPRODUCTION
ORDERINGINFORMATION
DGM-1040 or DGM-1080 are normally supplied with TTL "d~
glitch" input. For balanced ECl input, specify ECl Input on
P.O, or add suffix "ECl" to part number.
,.. COMPUTER
LABS
ANALOG DEVICES
.
. .
COMPUTER LABS. A DIVISION OF ANALOG DEVICES
505 EDWARDIA DRIVE. GREENSBORO.
N.C. 27409
(919) 292-6427
TWX 510-922-7954
ANALOG DEViCeS.
ROUTE 1 INDUSTRIAL PARK. NORWOOD, MA 02062
(617) 329-4700
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TWX 710-394-6577