LTC1741IFW View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1741IFW' PDF : 20 Pages View PDF
LTC1741
APPLICATIO S I FOR ATIO
SINGLE-ENDED
INPUT
2.35V ±1/2
RANGE
5V
+
1/2 LT1810
–
100Ω
500Ω
+
1/2 LT1810
–
500Ω
VCM
4.7µF
12pF
25Ω
25Ω AIN+
LTC1741
12pF
25Ω
25Ω AIN–
12pF
1741 F04
Figure 4. Differential Drive with Op Amps
Reference Operation
Figure 5 shows the LTC1741 reference circuitry consisting
of a 2.35V bandgap reference, a difference amplifier and
switching and control circuit. The internal voltage refer-
ence can be configured for two pin selectable input ranges
of 2V(±1V differential) or 3.2V(±1.6V differential). Tying
the SENSE pin to ground selects the 2V range; tying the
SENSE pin to VDD selects the 3.2V range.
The 2.35V 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 2.35V
reference output, VCM. This provides a high frequency low
impedance path to ground for internal and external cir-
cuitry. This is also the compensation capacitor for the
reference. It will not be stable without this capacitor.
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: REFHA and REFHB
for the high reference and REFLA and REFLB for the low
reference. The doubled output pins are needed to reduce
package inductance. Bypass capacitors must be con-
nected as shown in Figure 5.
2.35V
TIE TO VDD FOR 3.2V RANGE;
TIE TO GND FOR 2V RANGE;
RANGE = 2 • VSENSE FOR
1V < VSENSE < 1.6V
1µF
LTC1741
VCM
4.7µF
4Ω 2.35V BANDGAP
REFERENCE
1.6V 1V
SENSE
REFLB
0.1µF
REFHA
RANGE
DETECT
AND
CONTROL
BUFFER
INTERNAL ADC
HIGH REFERENCE
4.7µF
DIFF AMP
1µF
REFLA
0.1µF
REFHB
INTERNAL ADC
LOW REFERENCE
1741 F05
Figure 5. Equivalent Reference Circuit
Other voltage ranges in between the pin selectable ranges
can be programmed with two external resistors as shown
in Figure 6a. 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 since the logic threshold is close to ground and
VDD. The SENSE pin should be tied high or low 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.
Input Range
The input range can be set based on the application. For
oversampled signal processing in which the input fre-
quency is low (<10MHz), the largest input range will
provide the best signal-to-noise performance while main-
taining excellent SFDR. For high input frequencies
(> 40MHz), the 2V range will have the best SFDR perfor-
mance for the 2nd and 3rd harmonics, but the SNR will
degrade by 1.5dB. See the Typical Performance Charac-
teristics section.
1741f
13
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