ltc2226iuh-trpbf Linear Technology Corporation, ltc2226iuh-trpbf Datasheet - Page 19

• Manufacturer Part Number: ltc2226iuh-trpbf
• Description: Ltc2227 - 12-bit, 40msps Low Power 3v Adcs
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Reference Operation
Figure 9 shows the LTC2228/LTC2227/LTC2226 refer-
ence circuitry consisting of a 1.5V bandgap reference, a
difference amplifier and switching and control circuit. The
internal voltage reference can be configured 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 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.
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 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 applying
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.
U
U
CM
. This provides a high
W
CM
selects the 1V
U
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
LTC2228/27/26
V
CM
CONTROL
DETECT
4Ω
RANGE
AND
LTC2228/27/26
1.5V BANDGAP
DIFF AMP
REFERENCE
222876 F10
INTERNAL ADC
HIGH REFERENCE
INTERNAL ADC
LOW REFERENCE
1V
BUFFER
0.5V
19
222876fa
222876 F09