LTC1746 LINER [Linear Technology], LTC1746 Datasheet - Page 14

• Manufacturer Part Number: LTC1746
• Description: Low Power,14-Bit, 25Msps ADC
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
TIE TO V
LTC1746
Reference Operation
Figure 5 shows the LTC1746 reference circuitry consisting
of a 2.35V bandgap reference, a difference amplifier and
switching and control circuit. The internal voltage refer-
ence can be configured for two pin selectable input ranges
of 2V( 1V differential) or 3.2V( 1.6V differential). Tying
the SENSE pin to ground selects the 2V range; tying the
SENSE pin to V
The 2.35V bandgap reference serves two functions: its
output provides a DC bias point for setting the common
mode voltage of any external input circuitry; additionally,
the reference is used with a difference amplifier to gener-
ate the differential reference levels needed by the internal
ADC circuitry.
An external bypass capacitor of 4.7 F or larger is required
for the 2.35V reference output, V
frequency low impedance path to ground for internal and
external circuitry. This is also the compensation capacitor
for the reference. It will not be stable without this
capacitor.
14
TIE TO GND FOR 2V RANGE;
RANGE = 2 • V
DD
1V < V
FOR 3.2V RANGE;
SENSE
SENSE
Figure 5. Equivalent Reference Circuit
< 1.6V
2.35V
FOR
1 F
1 F
DD
selects the 3.2V range.
U
4.7 F
0.1 F
4.7 F
0.1 F
SENSE
REFHA
REFHB
REFLB
REFLA
V
U
CM
LTC1746
CONTROL
DETECT
4
RANGE
CM
AND
W
. This provides a high
2.35V BANDGAP
DIFF AMP
REFERENCE
INTERNAL ADC
HIGH REFERENCE
INTERNAL ADC
LOW REFERENCE
1.6V
U
BUFFER
1V
1746 F05
The difference amplifier generates the high and low refer-
ence for the ADC. High speed switching circuits are
connected to these outputs and they must be externally
bypassed. Each output has two pins: REFHA and REFHB
for the high reference and REFLA and REFLB for the low
reference. The doubled output pins are needed to reduce
package inductance. Bypass capacitors must be con-
nected as shown in Figure 5.
Other voltage ranges in between the pin selectable ranges
can be programmed with two external resistors as shown
in Figure 6a. An external reference can be used by applying
its output directly or through a resistor divider to SENSE.
It is not recommended to drive the SENSE pin with a logic
device since the logic threshold is close to ground and
V
the converter as possible. If the SENSE pin is driven
externally, it should be bypassed to ground as close to the
device as possible with a 1 F ceramic capacitor.
DD
. The SENSE pin should be tied high or low as close to
Figure 6b. 2.5V Range ADC with an External Reference
5V
0.1 F
4
Figure 6a. 2.2V Range ADC
LT1790-1.25
12.5k
11k
2.35V
1.1V
1, 2
6
SENSE
4.7 F
1 F
2.35V
V
1.25V
CM
LTC1746
SENSE
4.7 F
1 F
1746 F06a
V
CM
LTC1746
1746 F06b
1746f