LTC2351-12

13

235112fa

APPLICATIONS INFORMATION

SELECTING THE NUMBER OF CONVERTED CHANNELS

(SEL2, SEL1, SEL0)

These three control pins select the number of channels

being converted. 000 selects only the first channel (CH0)

for conversion. Incrementing SELx selects additional

channels for conversion, up to 6 channels. 101, 110 or

111 select all 6 channels for conversion. These pins must

be kept in a fixed state during conversion and during the

subsequent conversion to read data. When changing modes

between conversions, keep in mind that the output data

of a particular channel will remain unchanged until after

that channel is converted again. For example: convert

a sequence of 4 channels (CH0, CH1, CH2, CH3) with

SELx = 011, then, after these channels are converted

change SELx to 001 to convert just CH0 and CH1. See

Table 1. During the conversion of the first set of two

channels you will be able to read the data from the same

two channels converted as part of the previous group

of 4 channels. Later, you could convert 4 or more chan-

nels to read back the unread CH2 and CH3 data that was

converted in the first set of 4 channels. These pins are

often hardwired to enable the right number of channels

for a particular application. Choosing to convert fewer

channels per conversion results in faster throughput of

those channels. For example, 6 channels can be converted

at 250ksps/ch, while 3 channels can be converted at

500ksps/ch.

BIPOLAR/UNIPOLAR MODE

The input voltage range for each of the CHx input differ-

ential pairs is UNIPOLAR 0V – 2.5V when BIP is LOW, and

BIPOLAR ±1.25V when BIP is HIGH. This pin must be kept

in fixed state during conversion and during subsequent

conversion to read data. When changing BIP between con-

versions the full acquisition time must be allowed before

starting the next conversion. After changing modes from

BIPOLAR to UNIPOLAR, or from UNIPOLAR to BIPOLAR,

you can still read the first set of channels in the new mode,

by inverting the MSB to read these channels in the mode

that they were converted in.

DRIVING THE ANALOG INPUT

The differential analog inputs of the LTC2351-12 may be

driven differentially or as a single-ended input (i.e., the

stant. Any unwanted signal that is common to both inputs

of each input pair will be reduced by the common mode

rejection of the sample-and-hold circuit. The inputs draw

only one small current spike while charging the sample-

and-hold capacitors at the end of conversion. During

conversion, the analog inputs draw only a small leakage

Table 1. Conversion Sequence Control

(“acquire” represents simultaneous sampling of all channels; CHx represents conversion of channels)

SEL2

SEL1

SEL0

CHANNEL ACQUISITION AND CONVERSION SEQUENCE

0

0

0

acquire, CH0, acquire, CH0...

0

0

1

acquire, CH0, CH1, acquire, CH0, CH1...

0

1

0

acquire, CH0, CH1, CH2, acquire, CH0, CH1, CH2...

0

1

1

acquire, CH0, CH1, CH2, CH3, acquire, CH0, CH1, CH2, CH3...

1

0

0

acquire, CH0, CH1, CH2, CH3, CH4, acquire, CH0,CH1,CH2, CH3, CH4...

1

0

1

acquire, CH0, CH1, CH2, CH3, CH4, CH5, acquire, CH0, CH1, CH2, CH3, CH4, CH5...

1

1

0

acquire, CH0, CH1, CH2, CH3, CH4, CH5, acquire, CH0, CH1, CH2, CH3, CH4, CH5...

1

1

1

acquire, CH0, CH1, CH2, CH3, CH4, CH5, acquire, CH0, CH1, CH2, CH3, CH4, CH5...