Datasheet
LTC2238/LTC2237/LTC2236
18
223876fa
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
223876 F09
4Ω
DIFF AMP
1µF
1µF
INTERNAL ADC
LOW REFERENCE
1.5V BANDGAP
REFERENCE
1V
0.5V
RANGE
DETECT
AND
CONTROL
LTC2238/LTC2237/LTC2236
Figure 9. Equivalent Reference Circuit
APPLICATIO S I FOR ATIO
WUUU
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
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 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
CM
. This provides a high frequency
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.
Figure 10. 1.5V Range ADC
V
CM
SENSE
1.5V
0.75V
2.2µF
12k
1µF
12k
223876 F10
LTC2238
LTC2237
LTC2236