CN-0269

Circuit Note

Rev. 0 | Page 10 of 12

Figure 19. Errors vs. Switching Rate Without Front Buffer at 16-Channel

Single-Ended, 14 V Step

Figure 20. Errors vs. Switching Rate Without Input Buffer, 8-Channel

Differential Mode, 14 V Step

Figure 21. Errors vs. Switching Rate with Input Buffer, 16-Channel

Single-Ended Mode, 14 V Step

Figure 22. Errors vs. Switching Rate with Front Buffer, 8-Channel

Differential Mode, 14 V Step

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.

80

–100

–80

–60

–40

–20

0

20

40

60

1000

0

100

200

300

400

500

600

700

800

900

SWITCHING RATE (kHz)

CH 2, 4, 6… 16 SETTLE TO –7V

CH 1, 3, 5… 15 SETTLE TO +7V

1000

0

100

200

300

400

500

600

700

800

900

30

–40

–30

–20

–10

0

10

20

SWITCHING RATE (kHz)

CH 2, 4, 6, 8 SETTLE TO –7V

CH 1, 3, 5, 7 SETTLE TO +7V

1000

0

100

200

300

400

500

600

700

800

900

SWITCHING RATE (kHz)

25

–30

–25

–20

–15

–10

–5

0

5

10

15

20

CH 2, 4, 6, ...16 SETTLE TO –7V

CH 1, 3, 5, ... 15 SETTLE TO +7V

1000

0

100

200

300

400

500

600

700

800

900

SWITCHING RATE (kHz)

25

–30

–25

–20

–15

–10

–5

0

5

10

15

20

CH 2, 4, 6, 8 SETTLE TO –7V

CH1, 3, 5, 7 SETTLE TO +7V

1000

0

100

200

300

400

500

600

700

800

900

6

–6

–4

–2

0

2

4

SWITCHING RATE (kHz)

CH 1, 3, 5, 7 SETTLE TO +1V

CH 2, 4, 6, 8 SETTLE TO –1V