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dc1009af
DEMO MANUAL DC1009A
HARDWARE SETUP
Jumpers
JP1, JP2: Select the source for REF
+
and REF
–
, respectively.
REF
+
can be 5.00V from the onboard LT1236 reference
(default) or supplied externally. REF
–
can be ground (0V,
default) or supplied externally.
Connection To DC590 Serial Controller
J2 is the power and digital interface connector. Connect to
a DC590 serial controller with the supplied 14-conductor
ribbon cable. Digital signals are also connected to through-
hole test points on the circuit board.
Analog Connections
Analog signal connections are made via the row of turret
posts along the edge of the board.
GND: Ground turrets are connected directly to the internal
analog ground plane.
V
CC
: This is the supply for the ADC. Do not draw any
power from this point. External power may be applied to
this point after disabling the V
CC
supply on DC590. See
the DC590 quick start guide for details.
REF
+
, REF
–
: These turrets are connected to the LTC2486/
LTC2488/LTC2492 REF
+
and REF
–
pins. If the onboard
reference is being used, the reference voltage may be
monitored from this point. An external reference may be
connected to these terminals if JP1 and JP2 are configured
for external reference.
CH0-CH3: These are the differential inputs to the LTC2492/
LTC2488/LTC2486. They may be configured as single–
ended inputs with respect to the COM pin, or as differential
inputs (CH0-CH1, CH2-CH3) with the polarity software
selected.
EXPERIMENTS
Input Noise
Solder a short wire between the CH0 and CH1 turrets.
Connect the inputs to ground through a short wire and
start taking data. LTC2492 Noise should be approximately
0.12ppm of 5V (600nV
RMS
.) The electrical noise of the
LTC2488 is also 600nV
RMS
, however this is masked by
the 76.3µV quantization level. If the input is well between
code transitions, the noise level will read zero. If the input
is exactly between two codes such that the two adjacent
output codes have equal probability, the noise level will be
approximately 7.9ppm. The input noise of the LTC2486 is
apparent at very high gain settings (128 or 256.) Note that
with a 5V reference and gain set to 256, 1LSB is equal to
298nV which is lower than the 600nV
RMS
electrical noise
of the input stage.
Common Mode Rejection
Tie the two inputs (still connected together from previous
experiment) to ground through a short wire and note the
indicated voltage. Tie the inputs to REF
+
; the difference
should be less than 0.5V due to the 140dB+ CMRR of
the LTC2492. The LTC2488 will produce less than 1LSB
difference.
Input Normal Mode Rejection
The LTC2492 and LTC2486 SINC4 digital filter can be
software selected to reject 50Hz, 60Hz by 110dB, or both
50Hz and 60Hz by 87dB. The LTC2488’s SINC4 filter is fixed
at 50Hz/60Hz. To measure input normal mode rejection,
connect COM to a 2.5V source such as an LT1790-2.5
reference or a power supply. Connect any other input
(CH0-CH3) to the same supply through a 10k resistor.
Apply a 10Hz, 2V peak-to-peak sine wave to the input
through a 1µF capacitor.
Set the rejection frequency to 55Hz (LTC2492 only) and
start taking data. The input noise will be quite large, and
the graph of output vs time should show large variations.
Next, slowly increase the frequency to 55Hz. The noise
should be almost undetectable in the graph.
