Datasheet

LTC2282
15
2282fb
Reference Operation
Figure 9 shows the LTC2282 reference 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. Each
ADC channel has an independent reference with its own
bypass capacitors. The two channels can be used with
the same or different input ranges.
Other voltage ranges 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. For the best channel matching, connect
an external reference to SENSEA and SENSEB.
Input Range
The input range can be set based on the application.
The 2V input range will provide the best signal-to-noise
performance while maintaining excellent SFDR. The 1V
input range will have better SFDR performance, but the
SNR will degrade by 4dB. See the Typical Performance
Characteristics section.
Driving the Clock Input
The CLK inputs can be driven directly with a CMOS or TTL
level signal. A sinusoidal clock can also be used along with a
low jitter squaring circuit before the CLK pin (Figure 11).
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
2282 F09
4Ω
DIFF AMP
1μF
1μF
INTERNAL ADC
LOW REFERENCE
1.5V BANDGAP
REFERENCE
1V 0.5V
RANGE
DETECT
AND
CONTROL
LTC2282
Figure 9. Equivalent Reference Circuit
V
CM
SENSE
1.5V
0.75V
2.2μF
12k
1μF
12k
2282 F10
LTC2282
Figure 10. 1.5V Range ADC
APPLICATIONS INFORMATION