ltc1407a-1 Linear Technology Corporation, ltc1407a-1 Datasheet - Page 14

• Manufacturer Part Number: ltc1407a-1
• Description: Serial 12-bit, 3msps Simultaneous Sampling Adcs With Shutdown
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC1407-1/LTC1407A-1
INTERNAL REFERENCE
The LTC1407-1/LTC1407A-1 have an on-chip, tempera-
ture compensated, bandgap reference that is factory
trimmed near 2.5V to obtain a precise ±1.25V input span.
The reference amplifier output V
passed with a capacitor to ground. The reference ampli-
fier is stable with capacitors of 1µF or greater. For the best
noise performance, a 10µF ceramic or a 10µF tantalum in
parallel with a 0.1µF ceramic is recommended. The V
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
at 2.55V will see a DC quiescent load of 0.75mA and as
much as 3mA during conversion.
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
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
CH1
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
Integral nonlinearity errors (INL) and differential
14
+
–
, independent of the common mode voltage at either
minus CH0
3V REF
–
or the bipolar difference of CH1
U
REF
Figure 3
U
10µF
(Pin 3) and the voltage at the
DD
11
REF
3
. An external reference
W
V
GND
, (Pin 3) must be by-
LTC1407A-1
REF
LTC1407-1/
14071 F02
U
+
minus
REF
DD
.
nonlinearity 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 5. LTC1407-1/LTC1407A-1 Transfer Characteristic
011...111
011...110
011...101
100...010
100...001
100...000
–100
–120
–20
–40
–60
–80
0
–FS
100
Figure 4. CMRR vs Frequency
1k
INPUT VOLTAGE (V)
10k
FREQUENCY (Hz)
CH0
100k
1M
CH1
10M
FS – 1LSB
14071 G15
14071 F05
100M
14071fa