LTC2226 LINER [Linear Technology], LTC2226 Datasheet - Page 19

• Manufacturer Part Number: LTC2226
• Description: 12-Bit, 65/40/25Msps Low Power 3V ADCs
• Manufacturer: LINER [Linear Technology]

TYPICAL APPLICATIONS
Reference Operation
Figure 9 shows the LTC2228/LTC2227/LTC2226 refer-
ence circuitry consisting of a 1.5V bandgap reference,
a difference amplifi er and switching and control circuit.
The internal voltage reference can be confi gured for two
pin selectable input ranges of 2V (±1V differential) or 1V
(±0.5V differential). Tying the SENSE pin to V
the 2V range; tying the SENSE pin to V
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 amplifi er 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.
The difference amplifi er generates the high and low
reference 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 9.
Other voltage ranges in-between the pin selectable ranges
can be programmed with two external resistors as shown
in Figure 10. An external reference can be used by ap-
plying its output directly or through a resistor divider to
SENSE. It is not recommended to drive the SENSE pin
with a logic device. The SENSE pin should be tied to the
appropriate level as close to 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 1μF
ceramic capacitor.
CM
. This provides a high
CM
selects the 1V
DD
selects
TIE TO V
TIE TO V
LTC2228/LTC2227/LTC2226
RANGE = 2 • V
CM
0.5V < V
DD
FOR 2V RANGE;
FOR 1V RANGE;
2.2μF
1μF
1μF
1.5V
SENSE
SENSE
Figure 9. Equivalent Reference Circuit
< 1V
FOR
Figure 10. 1.5V Range ADC
12k
12k
0.75V
1.5V
2.2μF
SENSE
0.1μF
REFH
REFL
V
CM
SENSE
2.2μF
1μF
V
LTC2228/27/26
CM
CONTROL
4Ω
DETECT
RANGE
AND
LTC2228/27/26
DIFF AMP
1.5V BANDGAP
REFERENCE
222876 F10
INTERNAL ADC
HIGH REFERENCE
INTERNAL ADC
LOW REFERENCE
1V
BUFFER
0.5V
19
222876fb
222876 F09