Datasheet
LTC2225
12
2225fa
sample-and-hold charging glitches and limiting the
wideband noise at the converter input.
APPLICATIO S I FOR ATIO
WUUU
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.
Figure 5. Single-Ended Drive
Figure 4. Differential Drive with an Amplifier
25Ω
25Ω
12pF
2.2µF
V
CM
LTC2225
2225 F04
–
–
+
+
CM
ANALOG
INPUT
HIGH SPEED
DIFFERENTIAL
AMPLIFIER
A
IN
+
A
IN
–
25Ω
0.1µF
ANALOG
INPUT
V
CM
A
IN
+
A
IN
–
1k
12pF
2225 F05
2.2µF
1k
25Ω
0.1µF
LTC2225
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
DD
selects the 2V range; tying the SENSE
pin to V
CM
selects the 1V range.
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
CM
. This provides a high
frequency low impedance path to ground for internal and
external circuitry.
V
CM
REFH
SENSE
TIE TO V
DD
FOR 2V RANGE;
TIE TO V
CM
FOR 1V RANGE;
RANGE = 2 • V
SENSE
FOR
0.5V < V
SENSE
< 1V
1.5V
REFL
2.2µF
2.2µF
INTERNAL ADC
HIGH REFERENCE
BUFFER
0.1µF
2225 F06
LTC2225
4Ω
DIFF AMP
1µF
1µF
INTERNAL ADC
LOW REFERENCE
1.5V BANDGAP
REFERENCE
1V
0.5V
RANGE
DETECT
AND
CONTROL
Figure 6. Equivalent Reference Circuit
Figure 7. 1.5V Range ADC
V
CM
SENSE
1.5V
0.75V
2.2µF
12k
1µF
12k
2225 F07
LTC2225