QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 745
16-BIT DIFFERENTIAL INPUT DELTA SIGMA ADC
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LTC2433-1
DESCRIPTION
Demonstration circuit 745 features the LTC2433-1, a
16-bit high performance ∆Σ analog-to-digital con-
verter (ADC). The LTC2433-1 features 0.12 LSB line-
arity, 0.16 LSB full-scale accuracy, 5µV offset, and
1.45µV RMS noise. The inputs and reference are fully
differential, with input common mode rejection of 140
dB. The LTC2433-1 is available in a 10 pin MSOP
package and has an easy to use SPI interface.
DC745 is a member of Linear Technology‘s
QuickEval™ family of demonstration boards. It is de-
signed to allow easy evaluation of the LTC2433-1 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.
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 745
16-BIT DIFFERENTIAL INPUT DELTA SIGMA ADC
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QUICK START PROCEDURE
Connect DC745 to a DC590 USB Serial Controller us-
ing the supplied 14 conductor ribbon cable. Connect
DC590 to a 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.
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 745
16-BIT DIFFERENTIAL INPUT DELTA SIGMA ADC
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HARDWARE SET-UP
JUMPERS
JP1 – Select the source for REF+, either external or
5.00 volts from the onboard LT1236 reference (de-
fault.)
JP2 – Select the source for REF-, either external or
Ground (0 volts, default.)
JP3 – Trigger mode, either normal (default) or ex-
ternally triggered.
JP4 – Notch frequency. Connect to 60Hz for normal
operation (Fo = GND.) The LTC2433-1 is trimmed to
reject both 50Hz and 60Hz by at least 87dB when Fo
is grounded. Remove this jumper when supplying
an external clock to the Fo turret.
JP5 – Trigger input signal. Pin 1 is a 5 volt logic
signal, pin 2 is ground. When triggered mode is
selected on JP3, a rising edge starts a new conver-
sion. Note that a conversion in progress cannot be
stopped, so this signal can only be used to slow
down the rate at which samples are read from the
LTC2433-1. When Fo is grounded, the maximum
trigger frequency is 5Hz.
CONNECTION TO DC590 SERIAL CONTROLLER
J2 is the power and digital interface connector.
Connect to DC590 serial controller with supplied 14
conductor ribbon cable.
An external conversion clock may be applied to the
Fo turret to modify the frequency rejection charac-
teristics or data output rate of the LTC2433-1. Be
sure to remove JP4 before applying an external
clock. 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 LTC2433-1 data
sheet.
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 – 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+, REF- – These are connected to the LTC2433-
1 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 con-
nected to these terminals if JP1 and JP2 are con-
figured for external reference.
IN+, IN- – These are the differential inputs to the
LTC2433-1.
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 745
16-BIT DIFFERENTIAL INPUT DELTA SIGMA ADC
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EXPERIMENTS
INPUT NOISE
One of the characteristics of the LTC2433-1 is that
the 1.45µV input noise floor is far below the quanti-
zation level of 76uV when a 5-volt reference is used.
This means that the output will be stable if the input
noise level is also below 76uV. In this sense, the
LTC2433-1 is a true 16 effective bit part, whereas
many 16-bit SAR converters have several LSBs of
noise.
Solder a short wire from the IN- turret post to the
IN+ turret post. Noise should be below the quanti-
zation level of the LTC2433-1. This will result in a
noise reading of zero on the control software.
Applying an external reference of 100mV results in
a 1.52µV LSB, and the noise reading will be ap-
proximately 250ppm of 100mV, or 2.5µV. This is
slightly higher than the 1.45µV input thermal be-
cause quantization noise is also a factor.
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 1LSB due to the 130 dB CMRR
of the LTC2433-1.
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 one LSB of the first reading multiplied by –1.
One convenient voltage source for this experiment
is a single alkaline battery. While a battery has fairly
low noise, it is sensitive to temperature drift. It is
best to use a large (D-size) battery that is insulated
from air currents. A better source is a battery pow-
ered series reference such as the LT1790. This part
is available with output voltages of 1.25V, 2.048V,
2.5V, 3V, 3.3V, 4.096V and 5V.
INPUT NORMAL MODE REJECTION
The LTC2433-1’s SINC4 digital filter is trimmed to
strongly reject 50 or 60Hz line noise when operated
with the internal conversion clock. To measure in-
put normal mode rejection, connect IN- to a 2.5 volt
source such as an LT1790-2.5 reference or a power
supply. Apply a 10Hz, 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
LTC2433-1 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 60Hz. The
noise should be almost undetectable in the graph.
Note that the indicated noise in ppm may still be
above that of the datasheet specification because
the inputs are not connected to a DC source.
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 745
16-BIT DIFFERENTIAL INPUT DELTA SIGMA ADC
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