LTC2238 Linear, LTC2238 Datasheet - Page 18

• Manufacturer Part Number: LTC2238
• Description: (LTC2236 - LTC2238) LOW POWER 3V ADC
• Manufacturer: Linear

LTC2238/LTC2237/LTC2236
Reference Operation
Figure 9 shows the LTC2238/LTC2237/LTC2236 reference
circuitry consisting of a 1.5V bandgap reference, a differ-
ence amplifier and switching and control circuit. The inter-
nal voltage reference can be configured for two pin select-
able input ranges of 2V (±1V differential) or 1V (±0.5V dif-
ferential). Tying the SENSE pin to V
tying the SENSE 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 amplifier to generate the
differential reference levels needed by the internal ADC
circuitry. An external bypass capacitor is required for the
1.5V reference output, V
low impedance path to ground for internal and external
circuitry.
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. 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 applying
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.
APPLICATIO S I FOR ATIO
18
U
CM
CM
U
. This provides a high frequency
selects the 1V range.
DD
W
selects the 2V range;
U
TIE TO V
TIE TO V
RANGE = 2 • V
CM
0.5V < V
DD
FOR 2V RANGE;
FOR 1V RANGE;
2.2µF
1µF
1µF
1.5V
SENSE
SENSE
Figure 9. Equivalent Reference Circuit
< 1V
FOR
Figure 10. 1.5V Range ADC
12k
12k
0.75V
1.5V
2.2µF
SENSE
0.1µF
REFH
REFL
V
CM
SENSE
2.2µF
1µF
LTC2238/LTC2237/LTC2236
V
CONTROL
CM
DETECT
4Ω
RANGE
AND
LTC2238
LTC2237
LTC2236
1.5V BANDGAP
DIFF AMP
REFERENCE
223876 F10
INTERNAL ADC
HIGH REFERENCE
INTERNAL ADC
LOW REFERENCE
1V
BUFFER
0.5V
223876fa
223876 F09