LTC2225 LINER [Linear Technology], LTC2225 Datasheet - Page 12

• Manufacturer Part Number: LTC2225
• Description: 12-Bit, 10Msps Low Power 3V ADC
• Manufacturer: LINER [Linear Technology]

LTC2225
APPLICATIO S I FOR ATIO
sample-and-hold charging glitches and limiting the
wideband noise at the converter input.
Reference Operation
Figure 6 shows the LTC2225 reference circuitry consisting
of a 1.5V 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 1V (±0.5V differential). Tying the
SENSE pin to V
pin to V
The 1.5V 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 is required for
the 1.5V reference output, V
frequency low impedance path to ground for internal and
external circuitry.
12
ANALOG
ANALOG
INPUT
CM
INPUT
Figure 4. Differential Drive with an Amplifier
selects the 1V range.
0.1µF
DIFFERENTIAL
DD
CM
–
Figure 5. Single-Ended Drive
+
HIGH SPEED
AMPLIFIER
selects the 2V range; tying the SENSE
U
–
+
1k
U
25Ω
25Ω
1k
0.1µF
25Ω
25Ω
CM
. This provides a high
12pF
2.2µF
2.2µF
12pF
W
V
A
A
CM
IN
IN
V
A
A
+
–
CM
IN
IN
+
–
LTC2225
LTC2225
U
2225 F04
2225 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. The multiple output
pins are needed to reduce package inductance. Bypass
capacitors must be connected as shown in Figure 6.
Other voltage ranges in-between the pin selectable ranges
can be programmed with two external resistors as shown
in Figure 7. An external reference can be used by applying
its output directly or through a resistor divider to SENSE.
TIE TO V
TIE TO V
RANGE = 2 • V
CM
0.5V < V
DD
FOR 2V RANGE;
FOR 1V RANGE;
2.2µF
1µF
1µF
1.5V
SENSE
SENSE
Figure 6. Equivalent Reference Circuit
< 1V
FOR
Figure 7. 1.5V Range ADC
12k
12k
0.75V
1.5V
2.2µF
SENSE
0.1µF
REFH
REFL
V
CM
SENSE
2.2µF
1µF
LTC2225
V
CONTROL
CM
DETECT
4Ω
RANGE
AND
LTC2225
1.5V BANDGAP
DIFF AMP
REFERENCE
INTERNAL ADC
HIGH REFERENCE
INTERNAL ADC
LOW REFERENCE
2225 F07
1V
BUFFER
0.5V
2225 F06
2225fa