9

LTC1606

DC Performance

One way of measuring the transition noise associated with

a high resolution ADC is to use a technique where a DC

signal is applied to the input of the ADC and the resulting

output codes are collected over a large number of conver-

sions. For example in Figure 7, the distribution of output

code is shown for a DC input that has been digitized 4096

times. The distribution is Gaussian and the RMS code

transition is about 0.65LSB.

DIGITAL INTERFACE

Internal Clock

The ADC has an internal clock that is trimmed to achieve

a typical conversion time of 2.3

µs. No external adjust-

ments are required and, with the typical acquisition time of

1

µs, throughput performance of 250ksps is assured.

Timing and Control

Conversion start and data read are controlled by two

digital inputs: CS and R/C. To start a conversion and put

the sample-and-hold into the hold mode, bring CS and

R/C low for no less than 40ns. Once initiated, it cannot be

restarted until the conversion is complete. Converter

status is indicated by the BUSY output and this is low while

the conversion is in progress.

There are two modes of operation. The first mode is shown

in Figure 8. The digital input R/C is used to control the start

of conversion. CS is tied low. When R/C goes low, the

sample-and-hold goes into the hold mode and a conver-

sion is started. BUSY goes low and stays low during the

conversion and will go back high after the conversion has

been completed and the internal output shift registers

have been updated. R/C should remain low for no less than

40ns. During the time R/C is low, the digital outputs are in

a Hi-Z state. R/C should be brought back high within 1

µs

after the start of the conversion to ensure that no errors

occur in the digitized result. The second mode, shown in

Figure 9, uses the CS signal to control the start of a

conversion and the reading of the digital output. In this

mode the R/C input signal should be brought low no less

than 10ns before the falling edge of CS. The minimum

pulse width for CS is 40ns. When CS falls, BUSY goes low

and will stay low until the end of the conversion. BUSY will

go high after the conversion has been completed. The new

data is valid when CS is brought back low again to initiate

a read. Again, it is recommended that both R/C and CS

return high within 1

µs after the start of the conversion.

ACQUIRE

CONVERT

CONVERT

ACQUIRE

PREVIOUS

DATA VALID

PREVIOUS

DATA VALID

Hi-Z

NOT VALID

Hi-Z

DATA

VALID

DATA

VALID

R/C

BUSY

MODE

DATA MODE

1606 • F08

Figure 7. Histogram for 4096 Conversions

Figure 8. Conversion Timing with Outputs Enabled After Conversion (CS Tied Low)

2500

2000

1500

1000

500

0

–3

–2

–1

0

1

2

3

4

CODE

1606 • F07