CN0269 Datasheet, PDF (10/12 Pages) Analog Devices – Devices Connected

CN-0269
80
60
CH 2, 4, 6… 16 SETTLE TO –7V
40
20
0
–20
–40
CH 1, 3, 5… 15 SETTLE TO +7V
–60
–80
–100
0
100 200 300 400 500 600 700 800 900 1000
SWITCHING RATE (kHz)
Figure 19. Errors vs. Switching Rate Without Front Buffer at 16-Channel
Single-Ended, 14 V Step
30
CH 2, 4, 6, 8 SETTLE TO –7V
20
10
0
–10
–20
CH 1, 3, 5, 7 SETTLE TO +7V
–30
–40
0
100 200 300 400 500 600 700 800 900 1000
SWITCHING RATE (kHz)
Figure 20. Errors vs. Switching Rate Without Input Buffer, 8-Channel
Differential Mode, 14 V Step
25
CH 2, 4, 6, ...16 SETTLE TO –7V
20
15
10
5
0
–5
–10
–15
–20
CH 1, 3, 5, ... 15 SETTLE TO +7V
–25
–30
0
100 200 300 400 500 600 700 800 900 1000
SWITCHING RATE (kHz)
Figure 21. Errors vs. Switching Rate with Input Buffer, 16-Channel
Single-Ended Mode, 14 V Step
Circuit Note
25
20
CH 2, 4, 6, 8 SETTLE TO –7V
15
10
5
0
–5
–10
–15
CH1, 3, 5, 7 SETTLE TO +7V
–20
–25
–30
0
100 200 300 400 500 600 700 800 900 1000
SWITCHING RATE (kHz)
Figure 22. Errors vs. Switching Rate with Front Buffer, 8-Channel
Differential Mode, 14 V Step
6
CH 2, 4, 6, 8 SETTLE TO –1V
4
2
0
–2
–4
CH 1, 3, 5, 7 SETTLE TO +1V
–6
0 100 200 300 400 500 600 700 800 900 1000
SWITCHING RATE (kHz)
Figure 23. Errors vs. Switching Rate with Input Buffer, 8-Channel
Differential Mode, 2 V Step
From the figures above, we can see the circuit with the input
buffer has a better settling performance than the circuit without
front buffer at switching rates less than 1 MHz.
Figure 21, Figure 22, and Figure 23 show that with the input
buffer connected the circuit settles to 16 bits at channel-to-
channel switching rates up to 250 kHz.
COMMON VARIATIONS
The 18-bit AD7984 is available in a 10-lead MSOP or a 10-lead
QFN (LFCSP) package. There are a number of other PulSAR
ADCs available in the same package with 14-bit, 16-bit, and 18-
bit resolutions having various sampling rates.
Another possible choice for the buffer amplifiers is the AD8021.
If programmable gain is required, the AD8250, AD8251, and
AD8253 have 685 ns settling time to 0.001%. The ADG12xx
series of multiplexers can be used if lower capacitance is required.
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