Data Collection - Linear Technology DC2222A Demo Manual

Dc2222a setup
+
V
7
3
1
+
+
TO A
IN
8
U10
6
LTC2057-S8
2
–
5
R48, 10
4
–
V
R46, 4.99k
C65
0.047µF , 1206
+
V
7
3 1
+
–
TO A
IN
8
U18
6
LTC2057-S8
2
–
5
R51, 10
4
–
V
R50, 4.99k
C39
0.047µF , 1206
Figure 8a. LTC2508 Driver Circuit
+
V
7
3
1
+
+
TO A
IN
8
U10
LTC6202-S8
2
–
5
4
–
V
R114, 0
+
V
7
3
1
+
–
TO A
IN
8
U18
LTC6202-S8
2
–
5
4
–
V
R30, 0
Figure 8b. LTC2512 Driver Circuit
R9
0 , 0402
TO ADC
IN
C71 C90
4.7µF 4.7µF
1206 OPT
5040
C73 C91
4.7µF 4.7µF
1206 OPT
5040
R4
0 , 0402
TO ADC
IN
DC2222a F08a
6
R48, 10
TO ADC
+
IN
C71
6800pF
1206
C73
6
6800pF
1206
R51, 10
TO ADC
–
IN
DC2222a F08b
DEMO MANUAL DC2222A

Data Collection

For SINAD, THD or SNR testing a low noise, low distortion
differential output sine generator such as the Stanford
Research SR1 should be used. A low jitter RF oscillator
such as the Rohde & Schwarz SMB100A should be used
as the clock source.
This demo board is tested in house by taking an FFT of a
sine wave applied to the demo board's differential input.
+
This involves using a low jitter clock source, along with a
differential output sinusoidal generator at a frequency near
200Hz. The input signal level is approximately –1dBFS. The
input is level shifted and filtered with the circuit shown in
Figure 9. A typical FFT obtained with DC2222A is shown
in Figure 4. Note that to calculate the real SNR, the signal
level (F1 amplitude = –1dB) has to be added back to the
SNR that PScope displays. With the example shown in
Figure 4 this means that the actual SNR would be 123.54dB
–
instead of the 122.54dB that PScope displays. Taking
the RMS sum of the recalculated SNR and THD yields a
SINAD of 117.75dB. The THD shown was obtained using
the optional WIMA capacitors.
V
REF
SINE IN 1k
–V TO
REF
+V
REF
Figure 9. Differential Level Shifter
There are a number of scenarios that can produce mislead-
ing results when evaluating an ADC. One that is common
is feeding the converter with a frequency, that is a sub-
multiple of the sample rate, and which will only exercise
a small subset of the possible output codes. The proper
method is to pick an M/N frequency for the input sine
wave frequency. N is the number of samples in the FFT. M
is a prime number between one and N/2. Multiply M/N by
the sample rate to obtain the input sine wave frequency.
Another scenario that can yield poor results is if you
do not have a sine generator capable of ppm frequency
1k 1k
SINE IN 1k
+
A +V TO
IN REF
–V
REF
0.68µF
–
A
IN
0.68µF
DC2222a F09
dc2222af
7