LTC1746 Datasheet, PDF (14/20 Pages) Linear Technology – Low Power,14-Bit, 25Msps ADC

LTC1746
APPLICATIO S I FOR ATIO
Reference Operation
Figure 5 shows the LTC1746 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 V_DD 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 of 4.7µF or larger is required
for the 2.35V reference output, V_CM. This provides a high
frequency low impedance path to ground for internal and
external circuitry. 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.
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
V_DD. 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.
2.35V
TIE TO V_DD FOR 3.2V RANGE;
TIE TO GND FOR 2V RANGE;
RANGE = 2 • V_SENSE FOR
1V < V_SENSE < 1.6V
1µF
LTC1746
V_CM
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
1746 F05
Figure 5. Equivalent Reference Circuit
2.35V
12.5k
1.1V
11k
V_CM
4.7µF
SENSE ^LTC1746
1µF
1746 F06a
Figure 6a. 2.2V Range ADC
5V
0.1µF
2.35V
4
6 1.25V
LT1790-1.25
1, 2
V_CM
4.7µF
SENSE ^LTC1746
1µF
1746 F06b
Figure 6b. 2.5V Range ADC with an External Reference
1746f
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