QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 939
LTC2484
DESCRIPTION
Demonstration circuit 939 features the LTC2484, a
24-bit high performance
∆Σ
analog-to-digital con-
verter (ADC). The LTC2484 features 2ppm linearity,
0.5µV offset, and 600nV RMS noise. The input is fully
differential, with input common mode rejection of 140
dB. The LTC2484 is available in a 10 pin DFN package
and has an easy to use SPI interface.
DC939 is a member of Linear Technology‘s
QuickEval™ family of demonstration boards. It is de-
signed to allow easy evaluation of the LTC2484 and
may be connected directly to the target application’s
analog signals while using the DC590 USB Serial
Controller board and supplied software to measure
performance. The exposed ground planes allow
proper grounding to prototype circuitry. After evaluat-
ing with Linear Technology’s software, the digital sig-
nals can be connected to the end application’s proc-
essor / controller for development of the serial inter-
face.
Design files for this circuit board are available. Call
the LTC factory.
LTC is a trademark of Linear Technology Corporation
24-BIT DELTA SIGMA ADC
Figure 1. Proper Measurement Equipment Setup
Monitor reference or
external reference
source (See Hardware
Set-up)
Ribbon cable
To DC590
Controller
Differential
Input
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 939
24-BIT DELTA SIGMA ADC
QUICK START PROCEDURE
Connect DC939 to a DC590 USB Serial Controller us-
ing the supplied 14-conductor ribbon cable. Connect
DC590 to host PC with a standard USB A/B cable.
Run the evaluation software supplied with DC590 or
downloaded from
http://www.linear.com/software.
The correct program will be loaded automatically.
Click the COLLECT button to start reading the input
voltage. Details on software features are documented
in the control panel’s help menu.
Tools are available for logging data, changing refer-
ence voltage, changing the number of points in the
strip chart and histogram, and changing the number
of points averaged for the DVM display.
Figure 2. Software Screenshot
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 939
24-BIT DELTA SIGMA ADC
HARDWARE SET-UP
CONNECTION TO DC590 SERIAL CONTROLLER
J1 is the power and digital interface connector.
Connect to DC590 serial controller with supplied 14
conductor ribbon cable.
JUMPERS
JP1 – Select the source for REF+, either an
LT1236A-5 or externally supplied.
ANALOG CONNECTIONS
Analog signal connections are made via the row of
turret posts along the edge of the board. Also, when
connecting the board to an existing circuit the ex-
posed ground planes along the edges of the board
may be used to form a solid connection between
grounds.
GND – Three ground turrets are connected directly
to the internal ground planes.
VCC – This is the supply for the ADC. Do not draw
any power from this point.
REF+ - Connected to the LTC2484 reference pin. If
one of the onboard references is being used, the
reference voltage may be monitored from this point.
An external reference may be connected to these
terminals if JP1 is removed.
IN+, IN- – These are the differential inputs to the
LTC2484.
Fo - An external conversion clock may be applied to
the Fo turret to modify the frequency rejection char-
acteristics or data output rate of the LTC2484. This
should be a square wave with a low level equal to
ground and a high level equal to Vcc. While up to a
2MHz clock can be used, performance may be
compromised. Refer to the LTC2484 data sheet.
This terminal has a 5k pulldown resistor to keep it
from floating when no signal is applied.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 939
24-BIT DELTA SIGMA ADC
EXPERIMENTS
INPUT NOISE
The 600nV noise level of the LTC2484 inputs results
in 23 effective bits of resolution. Solder a short wire
from the IN- turret post to the IN+ turret post. This
allows measurement of the offset and noise level of
the LTC2484. The RMS noise display should ap-
proach 0.12ppm, assuming a 5V reference is used.
Note that many common “precision” voltage
sources have a higher noise level than 600nV, so it
is difficult to verify the 600nV resolution across the
entire input span. An alkaline D cell makes a con-
venient source, but it should be well insulated from
thermal and mechanical shock.
COMMON MODE REJECTION
Tie the two inputs (still connected together) 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 mini-
mum CMRR of the LTC2484.
BIPOLAR SYMMETRY
To demonstrate the symmetry of the ADCs transfer
function, connect a stable, low noise, floating volt-
age source (with a voltage less than Vref/2) from
IN+ to IN- and note the indicated voltage. Reverse
the polarity; the indicated voltage will typically be
within a few microvolts of the first reading multi-
plied by –1.
INPUT NORMAL MODE REJECTION
The LTC2484’s SINC4 digital filter is trimmed to re-
ject 50 or 60 Hz line noise when operated with the
internal conversion clock. To measure input normal
mode rejection, connect IN- to a 2.5 volt source
such as an LT1790-2.5 reference or 1k – 1k divider
from the onboard 5V reference to ground. Apply a
10 Hz, 2V peak-to-peak sine wave to IN+ through a
1uF capacitor. No DC bias is required because the
2-3MΩ input impedance of the LTC2484 tends to
self-bias the input to mid-reference (see datasheet
applications information for details.)
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 60 Hz (or
50Hz or 55Hz, depending on the selected rejection
frequency.) The noise should be almost undetect-
able in the graph. Note that the indicated noise in
ppm may still be above that of the datasheet speci-
fication because the inputs are not connected to a
DC source.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 939
24-BIT DELTA SIGMA ADC
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