LTC2283 Linear Technology, LTC2283 Datasheet - Page 15

• Manufacturer Part Number: LTC2283
• Description: 125Msps Low Power 3V ADC
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
Reference Operation
Figure 9 shows the LTC2283 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
pin to V
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.
TIE TO V
TIE TO V
RANGE = 2 • V
CM
0.5V < V
DD
CM
FOR 2V RANGE;
FOR 1V RANGE;
2.2μF
1μF
1μF
1.5V
SENSE
selects the 1V range.
SENSE
Figure 9. Equivalent Reference Circuit
< 1V
DD
FOR
selects the 2V range; tying the SENSE
2.2μF
SENSE
0.1μF
REFH
REFL
V
CM
LTC2283
CONTROL
4Ω
DETECT
RANGE
AND
CM
. This provides a high
DIFF AMP
1.5V BANDGAP
REFERENCE
INTERNAL ADC
HIGH REFERENCE
INTERNAL ADC
LOW REFERENCE
1V
BUFFER
0.5V
2283 F09
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).
Figure 10. 1.5V Range ADC
12k
12k
0.75V
1.5V
SENSE
2.2μF
1μF
V
CM
LTC2283
2283 F10
LTC2283
15
2283fb