4
DEMO MANUAL DC200
HIGH SPEED ADC
100kHz lowpass filter and works well for frequencies
below 100kHz. At the Nyquist frequency of 200kHz,
the TTE LE1182T-200k-400-720B 200kHz lowpass or
TTE Q70T-200k-30k-400-720B 200kHz bandpass
filters work well. The signal generator and filter should
produce < –96dB THD.
3. Adjust the magnitude of the input signal to within
10mV of negative and positive full scale. This ensures
that the maximum SINAD is achieved without the risk
of overdriving the input and producing unwanted
distortion. The conversion clock frequency can be set
within the range of 0kHz to 400kHz.
4. The conversion results can be observed in several
ways. The onboard LEDs indicate the state of each
QUICK START GUIDE
data bit. This is useful for giving a preliminary indica-
tion that the conversions are taking place and verifying
results when converting DC signals. The 14-bit paral-
lel output data is available on header J6. This allows
monitoring of each bit and can be connected to a logic
analyzer, DSP or oscilloscope. The data format is
two’s complement. Offset binary format is also avail-
able by using D13 instead of D13.
5. Dynamic performance can be measured by using an
FFT-based analyzer. By synchronizing the analog in-
put signal’s frequency to the conversion rate, or using
a windowing function, accurate SINAD, THD or other
dynamic characteristics can be evaluated.
OPERATIO
U
OPERATING THE BOARD
Powering the Board
To use the demo board, apply ±7V to ±15V at 200mA to
the banana jacks J1 and J3, and 0V (GND) to J2. Be careful
to observe the correct polarity. Internal regulators provide
±5V to the LTC1416/LTC1419. An LT1121-5 regulator
(U2) provides 5V for analog and digital circuitry; –5V is
provided for the A/D and buffer by the MC79L05 regulator
(U1).
The Analog Input
The LTC1416/LTC1419 have a unique feature not found on
previous ADCs: differential inputs with good common
mode rejection from DC to over 10MHz. Although this
feature is extremely valuable for rejecting noise and mea-
suring differential signals, the board can also be used to
evaluate the LTC1416/LTC1419 in single-ended mode
(with the “–” input grounded). This board allows evalua-
tion in either mode.
Differential (bipolar) analog signals are applied to the
LTC1416/LTC1419 demo board using BNC connectors J4
(noninverting + input) and J5 (inverting – input). The
analog signal input range is ±2.5V.
The LTC1416/LTC1419 A
IN
+
(noninverting) and A
IN
–
(inverting) inputs have a common mode range of V
SS
to
V
DD
. The full-scale differential between the signals applied
to A
IN
+
and A
IN
–
is ±2.5V. For example, when a 1.5V signal
is applied to the A
IN
–
input, the negative-to-positive full-
scale input range of A
IN
+
is –1V to 4V, corresponding to
an output code of 1000 0000 0000 to 0111 1111 1111.
The demo board is delivered with jumpers JP2 and JP4
closed. This configures the board for a ±2.5V input signal
centered around ground and applied to J4 ( A
IN
+
).
The board includes a recommended lowpass filter (R15
and R16, and C11) across the differential inputs. With the
component values shown, the cutoff frequency (f
S
) is:
1
= 1.56MHz
2π(102Ω)(1000pF)
These values can be altered to meet other circuit and input
signal requirements. For lower bandwidth input signals,
increase the value of C11. For undersampling applications
that take advantage of the input circuitry’s wide band-
width, decrease the capacitance of C11.
The best way to observe the performance of the LTC1416/
LTC1419 is to drive it directly from a low impedance signal
source. However, since some applications involve high
