LTC1405 Linear Technology, LTC1405 Datasheet - Page 7

• Manufacturer Part Number: LTC1405
• Description: 12-Bit/ 5Msps/ Sampling ADC
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Conversion Details
The LTC1405 is a high performance 12-bit A/D converter
that operates up to 5Msps. It is a complete solution with
an on-chip sample-and-hold, a 12-bit pipelined CMOS
ADC, a low drift programmable reference and an input
programmable gain amplifier. The digital output is paral-
lel, with a 12-bit two’s complement output and an out-of-
range (overflow) bit.
The rising edge of the CLK begins a conversion. The
differential analog inputs are simultaneously sampled and
passed on to the pipelined A/D. After two more conversion
starts (plus a 150ns conversion time) the digital outputs
are updated with the conversion result and will be ready for
capture on the third rising clock edge. Thus even though
a new conversion is begun every time CLK goes high, each
result takes three clock cycles to reach the output.
The analog signals that are passed from stage to stage in
the pipelined A/D are stored on capacitors. The signals on
these capacitors will be lost if the delay between conver-
sions is too long. For accurate conversion results, the part
should be clocked faster than 20kHz.
In some pipelined A/D converters if there is no clock
present, dynamic logic on the chip will droop and the
power consumption sharply increases. The LTC1405
doesn’t have this problem. If the part is not clocked for
1ms, an internal timer will refresh the dynamic logic. Thus
the clock can be turned off for long periods of time to save
power.
Power Supplies
The LTC1405 will operate from either a single 5V or dual
single or dual supply systems. The digital output drivers
have their own power supply pin (OV
from 3V to 5V, allowing direct connection to either 3V or
5V digital systems. For single supply operation, V
be connected to analog ground. For dual supply operation,
V
connected to a clean 5V analog supply. (Don’t connect V
to a noisy system digital supply.)
SS
5V supply, making it easy to interface the analog input to
should be connected to – 5V. Both V
U
U
W
DD
) which can be set
DD
pins should be
U
SS
should
DD
Analog Input Ranges
The LTC1405 has a flexible analog input with a wide
selection of input ranges. The input range is always
differential and is set by the voltages at the V
GAIN pins (Figure 1). The input range of the A/D core is
fixed at V
by the on-chip voltage reference or directly driven by an
external voltage. The GAIN pin is a digital input that
controls the gain of a preamplifier in the sample-and-hold
circuit. The gain of this PGA can be set to 1x or 2x. Table 1
gives the input range in terms of V
Table 1
GAIN PIN
5V (Logic H)
OV (Logic L)
Internal Reference
Figure 2 shows a simplified schematic of the LTC1405
reference circuitry. An on-chip temperature compensated
bandgap reference (V
The voltage at the V
to V
2.048V, which is connected to a reference amplifier. The
reference programming pin, SENSE, controls how the
reference amplifier drives the V
ground, the reference amplifier feedback is connected to
the R1/R2 voltage divider, thus making V
SENSE is tied to V
connected to SENSE thus making V
is tied to V
V
two additional resistors, V
between 2.048V and 4.5V.
REF
V
+
IN
–
GAIN
and V
+A
–A
V
REF
REF
IN
IN
/2. An internal voltage divider converts V
DD
REF
REF
, the reference amplifier is disconnected from
PGA
/2. The reference voltage, V
can be driven by an external voltage. With
1x/2x
Figure 1. Analog Input Circuit
REF
REF
PGA GAIN
, the reference amplifier feedback is
CM
pin sets the input span of the ADC
1x
2x
) is factory trimmed to 2.500V.
S/H
REF
REF
can be set to any voltage
REF
pin. If SENSE is tied to
REF
V
– V
– V
REF
= 2.048V. If SENSE
and GAIN.
(V
REF
REF
/2
LTC1405
INPUT RANGE
IN
REF
REF
/2 < V
/4 < V
= A
, is either set
IN
= 4.096V. If
REF
+
IN
IN
CORE
ADC
– A
< V
< V
and the
1405 F01
IN
REF
REF
CM
–
)
7
/2
/4
to