Datasheet

LTC2313-14
14
231314fa
For more information www.linear.com/LTC2313-14
applicaTions inForMaTion
buffer must be overdriven in the external reference con-
figuration with
a voltage 50mV higher than the nominal
reference output voltage in the internal configuration.
Using the Internal Reference
The internal bandgap and reference buffer are active by
default when the LTC2313-14 is not in sleep mode. The
reference voltage at the REF pin scales automatically with
the supply voltage at the V
DD
pin. The scaling of the refer-
ence voltage with supply is shown in Table 2.
Table 2. Reference Voltage vs Supply Range
SUPPLY VOLTAGE (V
DD
) REF VOLTAGE (V
REF
)
2.7V < V
DD
< 3.6V 2.048V
4.75V < V
DD
< 5.25V 4.096V
The reference voltage also determines the full-scale analog
input range of the LTC2313-14. For example, a 2.048V
reference voltage will accommodate an analog input range
from 0V to 2.048V. An analog input voltage that goes below
0V will be coded as all zeros and an analog input voltage
that exceeds 2.048V will be coded as all ones.
It is recommended that the REF pin be bypassed to ground
with a low ESR, 2.2µF ceramic chip capacitor for optimum
performance.
External Reference
An external reference can be used with the LTC2313-14
if better performance is required or to accommodate a
larger input voltage span. The only constraints are that
the external reference voltage must be 50mV higher than
Figure 11. LTC2313-14 Transfer Function Figure 12. Histogram for 16384 Conversions
the internal reference voltage (see Table 2) and must be
less than or equal to the supply voltage (or 4.3V for the 5V
supply range). For example, a 3.3V external reference may
be used with a 3.3V V
DD
supply voltage to provide a 3.3V
analog input voltage span (i.e. 3.3V > 2.048V + 50mV).
Or
alternatively, a 2.5V reference may be used with a 3V
supply voltage to provide a 2.5V input voltage range (i.e.
2.5V > 2.048V + 50mV). The LTC6655-3.3, LTC6655-2.5,
available from Linear Technology, may be suitable for
many applications requiring a high performance external
reference for either 3.3V or 2.5V input spans respectively.
Transfer Function
Figure 11 depicts the transfer function of the LTC2313-14.
The code transitions occur midway between successive
integer LSB values (i.e. 0.5LSB, 1.5LSB, 2.5LSB… FS-
0.5LSB). The output code is straight binary with 1LSB =
V
REF
/16,384.
DC Performance
The noise of an ADC can be evaluated in two ways:
signal-to-noise ratio (SNR) in the frequency domain and
histogram in the time domain. The LTC2313-14 excels
in both. The noise in the time domain histogram is the
transition noise associated with a 14-bit resolution ADC
which can be measured with a fixed DC signal applied
to the input of the ADC. The resulting output codes are
collected over a large number of conversions. The shape
of the distribution of codes will give an indication of the
magnitude of the transition noise. In Figure 12, the distri
-
bution of output codes is shown for a DC input that has
INPUT VOLTAGE (V)
OUTPUT CODE
231314 F11
111...111
111...110
000...000
000...001
FS – 1LSB0 1LSB
231314 F12
CODE
8194
COUNTS
8196 8197
8195
8198 8199 8200
σ = 0.7
0
2000
1000
3000
4000
5000
7000
6000