ltc1598l Linear Technology Corporation, ltc1598l Datasheet - Page 16

• Manufacturer Part Number: ltc1598l
• Description: 4- And 8-channel, 3v Micropower Sampling 12-bit Serial I/o A/d Converters
• Manufacturer: Linear Technology Corporation

LTC1594L/LTC1598L
APPLICATIONS
edge, the S & H goes into hold mode and the conversion
begins. The voltage on the “COM” input must remain
constant and be free of noise and ripple throughout the
conversion time. Otherwise, the conversion operation
may not be performed accurately. The conversion time is
12 CLK cycles. Therefore, a change in the “COM” input
voltage during this interval can cause conversion errors.
For a sinusoidal voltage on the “COM” input this error
would be:
Where f(“COM”) is the frequency of the “COM” input
voltage, V
frequency of the CLK. In most cases, V
significant. For a 60Hz signal on the “COM” input to
generate a 0.5LSB error (305 V) with the converter
running at CLK = 200kHz, its peak value would have to be
5.266mV.
ANALOG INPUTS
Because of the capacitive redistribution A/D conversion
techniques used, the analog inputs of the LTC1594L/
LTC1598L have capacitive switching input current spikes.
These current spikes settle quickly and do not cause a
problem. However, if large source resistances are used
or if slow settling op amps drive the inputs, care must be
taken to insure that the transients caused by the current
spikes settle completely before the conversion begins.
“Analog” Input Settling
The input capacitor of the LTC1594L/LTC1598L is switched
onto the selected channel input during the t
Figure 7) and samples the input signal within that time. The
sample phase is at least 1 1/2 CLK cycles before conver-
sion starts. The voltage on the “analog” input must settle
completely within t
improve the input settling time. If a large “analog” input
source resistance must be used, the sample time can be
increased by using a slower CLK frequency.
16
V
ERROR(MAX)
PEAK
= V
is its peak amplitude and f
SMPL
U
PEAK
. Minimizing R
(2 )(f)(“COM”)12/f
INFORMATION
U
W
SOURCE
ERROR
SMPL
CLK
+
CLK
and C1 will
will not be
U
time (see
is the
“COM” Input Settling
At the end of the t
“COM” input and conversion starts (see Figures 1 and 7).
During the conversion, the “analog” input voltage is
effectively “held” by the sample-and-hold and will not
affect the conversion result. However, it is critical that the
“COM” input voltage settles completely during the first
CLK cycle of the conversion time and be free of noise.
Minimizing R
If a large “COM” input source resistance must be used,
the time allowed for settling can be extended by using a
slower CLK frequency.
Input Op Amps
When driving the analog inputs with an op amp it is
important that the op amp settle within the allowed time
(see Figure 7). Again, the “analog” and “COM” input
sampling times can be extended as described above to
accommodate slower op amps. Most op amps, including
the LT
made to settle well even with the minimum settling
windows of 7.5 s (“analog” input) which occur at the
maximum clock rate of 200kHz.
Source Resistance
The analog inputs of the LTC1594L/LTC1598L look like a
20pF capacitor (C
a 90 channel resistance as shown in Figure 8. C
switched between the selected “analog” and “COM”
inputs once during each conversion cycle. Large external
source resistors and capacitances will slow the settling
of the inputs. It is important that the overall RC time
constants be short enough to allow the analog inputs to
completely settle within the allowed time.
V
IN
+
R
SOURCE
®
1006 and LT1413 single supply op amps, can be
+
“ANALOG”
Figure 8. Analog Input Equivalent Circuit
INPUT
SOURCE
C1
V
IN
–
SMPL
IN
R
90
) in series with a 1k resistor (R
ON
R
–
SOURCE
, the input capacitor switches to the
MUX
and C2 will improve settling time.
–
MUXOUT
C2
“COM”
INPUT
ADCIN
R
1k
ON
ON
LTC1594L
LTC1598L
IN
1594L/98L F08
15948lfb
) and
C
20pF
gets
IN