Datasheet

LTC1407-1/LTC1407A-1
15
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APPLICATIONS INFORMATION
INTERNAL REFERENCE
The LTC1407-1/LTC1407A-1 have an on-chip, temperature
compensated, bandgap reference that is factory trimmed
near 2.5V to obtain a precise ±1.25V input span. The ref-
erence amplifi er output V
REF
, (Pin 3) must be bypassed
with a capacitor to ground. The reference amplifier is
stable with capacitors of 1µF or greater. For the best noise
performance, a 10µF ceramic or a 10µF tantalum in paral-
lel with a 0.1µF ceramic is recommended. The V
REF
pin
can be overdriven with an external reference as shown
in Figure 3. The voltage of the external reference must be
higher than the 2.5V of the open-drain P-channel output
of the internal reference. The recommended range for an
external reference is 2.55V to V
DD
. An external reference
at 2.55V will see a DC quiescent load of 0.75mA and as
much as 3mA during conversion.
errors (DNL) are largely independent of the common
mode voltage. However, the offset error will vary. CMRR
is typically better than 60dB.
Figure 5 shows the ideal input/output characteristics for
the LTC1407-1/LTC1407A-1. The code transitions occur
midway between successive integer LSB values (i.e.,
0.5LSB, 1.5LSB, 2.5LSB, FS – 1.5LSB). The output code
is 2’s complement with 1LSB = 2.5V/16384 = 153µV for
the LTC1407A-1 and 1LSB = 2.5V/4096 = 610µV for the
LTC1407-1. The LTC1407A-1 has 1LSB RMS of Gaussian
white noise. Figure 6a shows the LTC1819 converting a
single-ended input signal to differential input signals for
optimum THD and SFDR performance as shown in the
FFT plot (Figure 6b).
Figure 4. CMRR vs Frequency
Figure 3
INPUT SPAN VERSUS REFERENCE VOLTAGE
The differential input range has a unipolar voltage span that
equals the difference between the voltage at the reference
buffer output V
REF
(Pin 3) and the voltage at the Exposed
Pad ground. The differential input range of ADC is –1.25V
to 1.25V when using the internal reference. The internal
ADC is referenced to these two nodes. This relationship
also holds true with an external reference.
DIFFERENTIAL INPUTS
The ADC will always convert the bipolar difference of CH0
+
minus CH0
–
or the bipolar difference of CH1
+
minus CH1
–
,
independent of the common mode voltage at either set
of inputs. The common mode rejection holds up at high
frequencies (see Figure 4). The only requirement is that
both inputs not go below ground or exceed V
DD
. Integral
nonlinearity errors (INL) and differential nonlinearity
LTC1407-1/
LTC1407A-1
V
REF
GND
14071 F02
3
11
10µF
3V REF
Figure 5. LTC1407-1/LTC1407A-1 Transfer Characteristic
FREQUENCY (Hz)
–80
CMRR (dB)
–40
0
–100
–60
–20
100 1k
14071 F04
–120
10k 100k 1M 10M 100M
CH0 CH1
INPUT VOLTAGE (V)
2’s COMPLEMENT OUTPUT CODE
14071 F05
011...111
011...110
011...101
100...000
100...001
100...010
FS – 1LSB–FS