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LTC2312-14

231214fa

For more information www.linear.com/LTC2312-14

APPLICATIONS INFORMATION

Exiting Nap/Sleep Mode

Waking up the LTC2312-14 from either nap or sleep

mode, as shown in Figures 8 and 9, requires SCK to be

pulsed one time. A conversion cycle (tACQ) may be started

immediately following nap mode as shown in Figure 8. A

period of time allowing the reference voltage to recover

mustfollowwakingupfromsleepmodeasshowninFigure

9. The wait period required before initiating a conversion

for the recommended value of CREF of 2.2µF is 1.1ms.

Power Supply Sequencing

The LTC2312-14 does not have any specific power sup-

ply sequencing requirements. Care should be taken to

observe the maximum voltage relationships described in

the Absolute Maximum Ratings section.

Single-Ended Analog Input Drive

The analog input of the LTC2312-14 is easy to drive. The

input draws only one small current spike while charging

the sample-and-hold capacitor following the falling edge

of CONV. During the conversion, the analog input draws

only a small leakage current. If the source impedance of

the driving circuit is low, then the input of the LTC2312-14

can be driven directly. As the source impedance increases,

so will the acquisition time. For minimum acquisition time

Figure 8. LTC2312-14 Entering/Exiting Nap Mode

Figure 9. LTC2312-14 Entering/Exiting Sleep Mode

with high source impedance, a buffer amplifier should be

used. The main requirement is that the amplifier driving

the analog input must settle after the small current spike

before the next conversion starts. Settling time must be

less than tACQ-MIN (700ns) for full performance at the

maximumthroughputrate.Whilechoosinganinputampli-

fier, also keep in mind the amount of noise and harmonic

distortion the amplifier contributes.

Choosing an Input Amplifier

Choosing an input amplifier is easy if a few requirements

are taken into consideration. First, to limit the magnitude

of the voltage spike seen by the amplifier from charging

the sampling capacitor, choose an amplifier that has a low

output impedance (<50Ω) at the closed-loop bandwidth

frequency. For example, if an amplifier is used in a gain

of 1 and has a unity-gain bandwidth of 50MHz, then the

output impedance at 50MHz must be less than 50Ω. The

second requirement is that the closed-loop bandwidth

must be greater than 50MHz to ensure adequate small

signal settling for full throughput rate. If slower op amps

are used, more time for settling can be provided by in-

creasing the time between conversions. The best choice

for an op amp to drive the LTC2312-14 will depend on the

application.Generally,applicationsfallintotwocategories:

AC applications where dynamic specifications are most

CONV

SCK

SDO

HI-Z STATE

231214 F08

NAP MODE

HOLD STATIC HIGH OR LOW

START tACQ

START tCONV

tACQ(MIN)

CONV

SCK

SDO

HI-Z STATE

231214 F09

NAP MODE

SLEEP MODE

HOLD STATIC HIGH OR LOW

VREF RECOVERY

tWAIT

START tCONV

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