ltc1411 Linear Technology Corporation, ltc1411 Datasheet - Page 11

• Manufacturer Part Number: ltc1411
• Description: Single Supply 14-bit 2.5msps Adc
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LT6203: Dual 100MHz, Low Noise, Low Power Op Amp.
Specified at 3V, 5V and 5V supplies. 1.9nV/ Hz noise
voltage.
Programmable Input Range
The LTC1411 has two logic input pins (PGA0 and PGA1)
that are used to select one of four analog input ranges.
These input ranges are set by changing the reference
voltage that is applied to the internal DAC of the ADC
(REFCOM2). For the “0dB” setting the internal DAC sees
the full reference voltage of 4V. The analog input range is
0.7V to 4.3V with A
span of 1.8V with respect to the voltage applied to A
. For the “– 3dB” setting the internal reference is reduced
to 0.707 • 4V = 2.9V. Likewise the input span is reduced
to 1.28V. The following table lists the input span with
respect to A
Table 1. Input Spans for LTC1411
When changing from one input span to another, more time
is needed for the REFCOM2 pin to reach the correct level
because the bypass capacitor on the pin needs to be charged
or discharged. Figure 6 shows the recommended capaci-
tors at the REFCOM1 and REFCOM2 pins (10 F each).
When – 6dB or – 9dB is selected, the voltage at REFCOM1
(see Figure 2) must first settle before REFCOM2 reaches
the correct level. The typical delay is about 700ms.
When the REFCOM2 level is changed from 2.9V to 4V
(changing PGA setting from – 3dB to 0dB), the typical delay
is 0.6ms. However, if the voltage at REFCOM2 is changed
from 4V to 2.9V (changing PGA setting from 0dB to – 3dB)
only a 60 A sink current is present to discharge the 10 F
bypass capacitor. In this case, the delay will be 11ms.
PGA0
5V
5V
0V
0V
IN
–
PGA1
5V
0V
5V
0V
for the different PGA0 and PGA1 settings.
IN
U
–
= 2.5V. This corresponds to an input
LEVEL
– 3dB
– 6dB
– 9dB
0dB
U
W
INPUT
SPAN
1.28V
0.64V
1.8V
0.9V
REFCOM2
U
VOLTAGE
1.45V
2.9V
4V
2V
IN
–
Internal Reference
The LTC1411 has an on-chip, temperature compensated,
curvature corrected, bandgap reference that is factory
trimmed to 2.500V. If this REFOUT pin is used to drive the
A
this REFOUT voltage sets the bipolar zero for the ADC.
The REFIN pin is connected to the reference buffer through
a 2k resistor and two PGA switches. The REFIN pin can be
connected to REFOUT directly or to an external reference.
Figure 6 shows the reference and buffer structure for the
LTC1411. The input to the reference buffer is either REFIN
or 1/2 of REFIN depending on the PGA selection. The
REFCOM1 pin bypassed with a 10 F tantalum capacitor
helps reduce the noise going into the buffer. The reference
buffer has a gain of 1.62 or 1.15 (depends on PGA
selection). It is compensated at the REFCOM2 pin with a
10 F tantalum capacitor. The input span of the ADC is set
by the output voltage of this REFCOM2 voltage. For a 2.5V
input at the REFIN pin, the REFCOM2 will have 4V output
for PGA1 = PGA0 = 5V and the ADC will have a span of 3.6V.
IN
–
pin, a 22 F tantalum bypass capacitor is required and
**
*
THIS PIN CAN BE TIED TO REFOUT OR AN EXTERNAL SOURCE
A 22 F CAPACITOR IS NEEDED IF REFOUT IS USED TO DRIVE A
Figure 6. Reference Structure for the LTC1411
for PGA1 = PGA0 = 5V
22 F**
10 F
10 F
REFCOM1
REFCOM2
REFOUT
REFIN*
5k
5k
REFERENCE
X1.62
BANDGAP
2.5V
1411 F06
2k
LTC1411
IN
11
–
1411f