Datasheet

LTC2315-12
15
231512fa
For more information www.linear.com/LTC2315-12
Figure 11. LTC2315-12 Transfer Function
INPUT VOLTAGE (V)
OUTPUT CODE
231512 F11
111...111
111...110
000...000
000...001
FS – 1LSB0 1LSB
APPLICATIONS INFORMATION
ADC Reference
A low noise, low temperature drift reference is critical to
achieving the full data sheet performance of the ADC. The
LTC2315-12 provides an excellent internal reference with
a guaranteed 20ppm/°C maximum temperature coefficient.
For added flexibility, an external reference may also be used.
The high speed, low noise internal reference buffer is used
only in the internal reference configuration. The reference
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 LTC2315-12 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 the following table.
Table 2: Reference Voltage vs Supply Range
SUPPLY VOLTAGE (V
DD
) REF VOLTAGE (V
REF
)
2.7V –> 3.6V 2.048V
4.75V –> 5.25V 4.096V
The reference voltage also determines the full-scale analog
input range of the LTC2315-12. 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 LTC2315-12
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
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 LTC2315-12.
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
/4,096.
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 LTC2315-12 excels in both. The noise in
the time domain histogram is the transition noise associated