Datasheet

3
dc1012af
DEMO MANUAL DC1012A
EXPERIMENTS
HARDWARE SETUP
GND – Ground turrets are connected directly to the internal
analog ground plane.
PWR GND – Power ground, connected to the power
return trace.
VCC – 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
LTC2495/LTC2497/LTC2499 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 JP3 are configured for external reference.
Note: The REF+ and REF– terminals are decoupled to
ground with 0.01μF and 4.7μF capacitors in parallel. Thus
any source connected to these terminals must be able to
drive a capacitive load and have very low impedance at
DC. Examples are series references that require an out-
put capacitor and C-Load™ stable op amps such as the
LT
®
1219 and LT1368.
CH0 – CH15 – These are the differential inputs to the
LTC2499/2497/2495. They may be configured as single-
ended inputs with respect to the
COM pin, or as differential
inputs (CH0-1, CH2-3, etc.) with the polarity software
selected.
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. LTC2499 noise should be approximately
0.12ppm of 5V (600nV
RMS
.) The electrical noise of the
LTC2497 is also 600nV
RMS
, however this is masked by the
76.3µV quantization
level. If the input is midway between
code transitions, the noise level will read zero. If the input
is exactly on a code transition such that the two adjacent
output codes have equal probability, the noise level will
be approximately 7.9ppm. The input noise of the LTC2495
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 0.5μV due to the 140dB+ CMRR of
the LTC2499. The LTC2497 will produce less than 1
LSB
difference.
Input Normal Mode Rejection
The LTC2499 and LTC2495 SINC4 digital filter can be
software selected to reject 50Hz, 60Hz by 110dB, or both
50Hz and 60Hz by 87dB. The LTC2497’s SINC4 filter is
fixed at 50Hz/60Hz. To measure input normal mode rejec-
tion, connect COM to a 2.5V source such as an LT1790
-2.5
reference or a power supply. Connect any other input
(CH0-CH15) 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 (LTC2499 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.