Datasheet

ADS7813
8
SBAS043C
www.ti.com
STARTING A CONVERSION
If a conversion is not currently in progress, a falling edge on
the CONV input places the sample and hold into the hold
mode and begins a conversion, as shown in Figure 2 and
with the timing given in Table II. During the conversion, the
CONV input is ignored. Starting a conversion does not
depend on the state of CS. A conversion can be started once
every 25µs (40kHz maximum conversion rate). There is no
minimum conversion rate.
Even though the CONV input is ignored while a conversion
is in progress, this input should be held static during the
conversion period. Transitions on this digital input can
easily couple into sensitive analog portions of the converter,
adversely affecting the conversion results (see the Sensitiv-
ity to External Digital Signals section of this data sheet for
more information).
Ideally, the CONV input should go LOW and remain LOW
throughout the conversion. It should return HIGH sometime
after BUSY goes HIGH. In addition, it should be HIGH
prior to the start of the next conversion for a minimum time
period given by t
5
. This will ensure that the digital transition
on the CONV input will not affect the signal that is acquired
for the next conversion.
An acceptable alternative is to return the CONV input HIGH
as soon after the start of the conversion as possible. For
example, a negative going pulse 100ns wide would make a
good CONV input signal. It is strongly recommended that
from time t
2
after the start of a conversion until BUSY rises,
the CONV input should be held static (either HIGH or
LOW). During this time, the converter is more sensitive to
external noise.
TABLE II. ADS7813 Timing. T
A
= –40°C to +85°C.
SYMBOL DESCRIPTION MIN TYP MAX UNITS
t
1
Conversion Plus Acquisition Time 25 µs
t
2
CONV LOW to All Digital 8 µs
Inputs Stable
t
3
CONV LOW to Initiate a Conversion 40 ns
t
4
BUSY Rising to Any Digital 0 ns
Input Active
t
5
CONV HIGH Prior to Start 2 µs
of Conversion
t
6
BUSY LOW 19 20 µs
t
7
CONV LOW to BUSY LOW 85 120 ns
t
8
Aperture Delay 40 ns
t
9
Conversion Time 18 20 µs
t
10
Conversion Complete to 1.1 2 µs
BUSY Rising
t
11
Acquisition Time 5 µs
t
12
CONV LOW to Rising Edge 1.4 µs
of First DATACLK
t
13
Internal DATACLK HIGH 250 350 500 ns
t
14
Internal DATACLK LOW 600 760 875 ns
t
15
Internal DATACLK Period 1.1 µs
t
16
DATA Valid to Internal 20 ns
DATACLK Rising
t
17
Internal DATACLK Falling 400 ns
to DATA Not Valid
t
18
Falling Edge of Last DATACLK 800 ns
to BUSY Rising
t
19
External DATACLK Rising 15 ns
to DATA Not Valid
t
20
External DATACLK Rising 55 85 ns
to DATA Valid
t
21
External DATACLK HIGH 50 ns
t
22
External DATACLK LOW 50 ns
t
23
External DATACLK Period 100 ns
t
24
CONV LOW to External 100 ns
DATACLK Active
t
25
External DATACLK LOW 2 µs
or CS HIGH to BUSY Rising
t
26
CS LOW to Digital Outputs Enabled 85 ns
t
27
CS HIGH to Digital Outputs Disabled 85 ns
Convert ConvertAcquire Acquire
MODE
BUSY
CONV
t
3
t
2
t
1
t
5
t
4
t
7
t
6
t
8
t
10
t
11
t
9
FIGURE 2. Basic Conversion Timing.