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db09284e_V3.01_preliminary_2017-07-06

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AS89020

V3.01

5.6 Input amplifier

The input amplifier has a low-pass characteristic. The bandwidth of amplifiers depends on the selected gain

via the registers SW_RX (see Table 3). Therefore the bandwidth has a significant influence of value which

is sampled and converted. The bandwidth determines the settling time tSET which is required from the input

amplifier to provide a stable signal for an accuracy of 12 bits at its output. The time tSET can be estimated

with following formula.

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9

2 ∗ ������ ∗ ������3������������

tSET: settling time to achieve 12 bit accuracy

f3dB: Bandwidth of input amplifier (see Table 3)

Table 5 shows the values for the settling time tSET for the individual gain stages. These values are calculated

with the minimal bandwidth of input amplifier.

Table 5: settling time tSET

STAGE (see Table 11)

MIN. BANDWIDTH

SETTLING TIME tSET

0

170

kHz

8.4

µs

1

100

kHz

14.3

µs

2

50

kHz

28.6

µs

3

40

kHz

35.8

µs

4

25

kHz

57.3

µs

5

15

kHz

95.5

µs

6

12

kHz

119.4

µs

7

8

kHz

179.0

µs

8

4

kHz

358.1

µs

9

2

kHz

716.2

µs

10-15

1.2

kHz

1193.7

µs

16-20

20

kHz

71.6

µs

21

12

kHz

119.4

µs

22

10

kHz

143.2

µs

23

6

kHz

238.7

µs

24

3

kHz

477.5

µs

Figure 6 shows the effect of settling time tSET on the conversion result. For the AD conversion the actual

value is sampled from output of the input amplifier by track and hold circuit. The conversion result always

corresponds to the low-pass characteristic of the input amplifier. This is especially taken into account when

the sample taken inside the settling time tSET or frequency of the input signal is close to the bandwidth of

input amplifier. The AD conversion takes place at an accuracy of 12 Bits, regardless of the bandwidth of

input amplifier.