Datasheet
LTC2228/LTC2227/LTC2226
19
222876fb
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
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 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
CM
. This provides a high
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.
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
222876 F09
LTC2228/27/26
4Ω
DIFF AMP
1μF
1μF
INTERNAL ADC
LOW REFERENCE
1.5V BANDGAP
REFERENCE
1V 0.5V
RANGE
DETECT
AND
CONTROL
Figure 9. Equivalent Reference Circuit
V
CM
SENSE
1.5V
0.75V
2.2μF
12k
1μF
12k
222876 F10
LTC2228/27/26
Figure 10. 1.5V Range ADC