Datasheet
5
dc846af
DEMO MANUAL DC846A
EXPERIMENTS
INPUT NOISE
Solder a short wire from the CH0 to CH1. Ensure that the
buffer amplifiers are in their active region of operation by
either biasing the inputs to mid-supply with a 10k to 10k
divider when the buffer amplifier is powered from V
CC
and ground, or tie the inputs to ground and connect an
external 7.5V/–2.5V supply to the AMPV- turret (JP1 and
JP2 must be set to EXT.)
Set the demo software to OSR32768 (6.8 samples per sec-
ond) and check the 2× box. Noise should be approximately
0.04ppm of V
REF
(200nV.) Next, select different oversample
ratios. Measured noise for each oversample ratio should
be close to values given in the LTC2447 data sheet.
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 5V due to the 120dB CMRR of the
LTC2447.
This experiment requires an external power supply to the
buffer amplifier.
If the common mode voltage is limited to GND +0.25V to
V
CC
-0.25V, this test may be performed with the amplifier
supplies set to ground and V
CC
.
INPUT NORMAL MODE REJECTION
The LTC2447’s SINC4 digital filter is trimmed to strongly
reject both 50Hz and 60Hz line noise when operated with
the internal conversion clock and oversample ratio 32768
(6.8 samples per second.) 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 to 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.
Select OSR32768 (6.8 samples per second) and 2× mode
in the demo software 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. Note that the
indicated noise in ppm may still be above that of the data
sheet specification because the inputs are not connected
to a DC source.
Change the OSR to 16384 (13.75 samples per second); the
noise will increase substantially, as the first notch at this
OSR is at 110Hz. Increase the signal generator frequency
to 110Hz, the noise will drop again.