LTC2436-1
26
24361f
APPLICATIO S I FOR ATIO
WUUU
Figure 28. Resolution (INL
MAX
≤ 1LSB)
vs Output Data Rate and Temperature
Figure 29. Offset Error vs Output
Data Rate and Reference Voltage
OUTPUT DATA RATE (READINGS/SEC)
0 102030405060708090100
OFFSET ERROR (LSB)
24361 F29
16
8
0
V
REF
= 5V
V
CC
= 5V
REF
+
= GND
V
INCM
= 2.5V
V
IN
= 0V
F
O
= EXTERNAL OSCILLATOR
T
A
= 25°C
V
REF
= 2.5V
OUTPUT DATA RATE (READINGS/SEC)
0 102030405060708090100
RESOLUTION (BITS)
24361 F28
18
16
14
12
10
8
T
A
= 85°C
V
CC
= 5V
REF
+
= 5V
REF
–
= GND
V
INCM
= 2.5V
–2.5V < V
IN
< 2.5V
F
O
= EXTERNAL OSCILLATOR
RESOLUTION = LOG
2
(V
REF
/INL
MAX
)
T
A
= 25°C
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.
Increasing Input Resolution by Reducing Reference
Voltage
The resolution of the LTC2436-1 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 LTC2436-1 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 LTC2436-1 has a 76µV LSB when used with a 5V
reference, however the thermal noise of the inputs is
800nV
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 800nV
RMS
. A 325mV reference corresponds to
a 5µV LSB, which is approximately the peak-to-peak value
of the 800nV
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 800nV
RMS
.
Figure 30 shows two methods of dividing down the
reference voltage to the LTC2436-1. 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 LTC2436-
1 from a 5V source. The resulting ±49mV input range and
1.5µV 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 LTC2436-1 from a 5V source. The fully
differential property of the LTC2436-1 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.
