Linear Technology LTC2439CGN-1, 8-Channel Differential 16-Bit ADC (req DC590) DC790A DC790A Fiche De Données

LTC2439-1

26

24391fa

First, a change in f

EOSC

 will result in a proportional change

in the internal notch position and in a reduction of the
converter differential mode rejection at the power line
frequency. In many applications, the subsequent perfor-
mance degradation can be substantially reduced by rely-
ing upon the LTC2439-1’s exceptional common mode
rejection and by carefully eliminating common mode to
differential mode conversion sources in the input circuit.
The user should avoid single-ended input filters and
should maintain a very high degree of matching and
symmetry in the circuits driving the IN

+

 and IN

–

 pins.

Second, the increase in clock frequency will increase
proportionally the amount of sampling charge transferred
through the input and the reference pins. If large external
input and/or reference capacitors (C

IN

, C

REF

) are used, the

previous section provides formulae for evaluating the
effect of the source resistance upon the converter perfor-
mance for any value of f

EOSC

. If small external input and/

or reference capacitors (C

IN

, C

REF

) are used, the effect of

the external source resistance upon the LTC2439-1 typical
performance can be inferred from Figures 14, 15, 19 and
20 in which the horizontal axis is scaled by 139,800/f

EOSC

.

Third, an increase in the frequency of the external oscilla-
tor above 460800Hz (a more than 3

× increase in the output

data rate) will start to decrease the effectiveness of the
internal autocalibration circuits. This will result in a pro-
gressive degradation in the converter accuracy and linear-
ity. Typical measured performance curves for output data
rates up to 100 readings per second are shown in Fig-
ures 24, 25, 26, 27, 28 and 29. In order to obtain the
highest possible level of accuracy from this converter at
output data rates above 20 readings per second, the user
is advised to maximize the power supply voltage used and
to limit the maximum ambient operating temperature. In
certain circumstances, a reduction of the differential refer-
ence voltage may be beneficial.

The resolution of the LTC2439-1 can be increased by
reducing the reference voltage. It is often necessary to

amplify low level signals to increase the voltage resolution
of ADCs that cannot operate with a low reference voltage.
The LTC2439-1 can be used with reference voltages as low
as 100mV, corresponding to a 

±50mV input range with full

16-bit resolution. Reducing the reference voltage is func-
tionally equivalent to amplifying the input signal, however
no amplifier is required.

The LTC2439-1 has a 76

µV LSB when used with a 5V

reference, however the thermal noise of the inputs is
1

µV

RMS

 and is independent of reference voltage. Thus

reducing the reference voltage will increase the resolution
at the inputs as long as the LSB voltage is significantly
larger than 1

µV

RMS

. A 325mV reference corresponds to a

5

µV LSB, which is approximately the peak-to-peak value

of the 1

µV

RMS

 input thermal noise. At this point, the output

code will be stable to 

±1LSB for a fixed input. As the

reference is decreased further, the measured noise will
approach 1

µV

RMS

.

Figure 30 shows two methods of dividing down the
reference voltage to the LTC2439-1. Where absolute accu-
racy is required, a precision divider such as the Vishay
MPM series dividers in a SOT-23 package may be used. A
51:1 divider provides a 98mV reference to the LTC2439-1
from a 5V source. The resulting 

±49mV input range and

1.5

µV LSB is suitable for thermocouple and 10mV full-

scale strain gauge measurements.

If high initial accuracy is not critical, a standard 2%
resistor array such as the Panasonic EXB series may be
used. Single package resistor arrays provide better tem-
perature stability than discrete resistors. An array of eight
resistors can be configured as shown to provide a 294mV
reference to the LTC2439-1 from a 5V source. The fully
differential property of the LTC2439-1 reference terminals
allow the reference voltage to be taken from four central
resistors in the network connected in parallel, minimizing
drift in the presence of thermal gradients. This is an ideal
reference for medium accuracy sensors such as silicon
micromachined pressure and force sensors. These de-
vices typically have accuracies on the order of 2% and full-
scale outputs of 50mV to 200mV.