Analog Input; Component Selection - Linear Technology DC1805A Series Manual

DEMO MANUAL DC1805A
Dc1805a setup
DC Power
The DC1805A requires 9VDC and draws less than 70mA.
Most of the supply current is consumed by the CPLD,
regulators and discrete logic on the board. The 9VDC input
voltage powers the LTC6362 and the ADC through LT
regulators which provide protection against accidental
reverse bias. Additional regulators provide power for the
CPLD. See Figure 1 for connection details.
Clock Source
You must provide a low jitter 3.3V
square wave to J1. The clock input is AC-coupled so the
DC level of the clock signal is not important. A genera-
tor like the Marconi Instruments, Multisource Generator
2026, HP8644 or the DC1216A-A is recommended. Even
a good generator can start to produce noticeable jitter at
low frequencies. Therefore it is recommended for lower
sample rates to divide down a higher frequency clock
to the desired sample rate. The ratio of clock frequency
to conversion rate is 62:1 for 18-bit parts and 50:1 for
16-bit parts. If the clock input is to be driven with logic,
it is recommended that the 50Ω terminator (R1) be re-
moved. Slow rising edges may compromise the SNR of
the converter in the presence of high amplitude higher
frequency input signals.
A +
IN
4
maximum sine or
P-P
R17
1k
R18
35.7Ω
R15
1k
+
R26
LTC6362
1k
–
R28
35.7Ω
R27
1k
Figure 3. LTC6362 Ground Referenced Single-Ended to Differential Converter
Reference
The default reference is a LTC6655 5V reference. An external
reference can be used by removing (U3) and populating
(R6) with 0 resistor. If an external reference is used it must
1763 settle quickly in the presence of glitches on the REF pin.
®

Analog Input

LTC6362 drives the analog input of the LTC2379-18 on
the DC1805A as shown in Figure 3. This circuit converts
a single-ended input signal to a differential output signal
applied at the ADC inputs. Please refer to the LTC6362 data
sheet for various configurations of the LTC6362 interface
to the SAR ADC.
AC-coupling the input may degrade the distortion per-
formance of the ADC due to nonlinearity of the coupling
capacitor.

Component Selection

When driving a low noise, low distortion ADC such as
the LTC2379-18 with the LTC6362, component selection
is important so as to not degrade performance. Resistors
should have low values to minimize noise and distortion.
Metal film resistors are recommended to reduce distortion
caused by self heating. To further reduce distortion, NPO
or silver mica capacitors should be used because of their
low voltage coefficients.
C27
R19
NPO
0Ω
3900pF
0402
C26
NPO
R19
3900pF
0Ω
0402
C29
NPO
3900pF
+
LTC2379-18
–
DC1805A F03
dc1805af