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AD7608

Data Sheet

Rev. A | Page 26 of 32

DIGITAL FILTER

The AD7608 contains an optional digital first-order sinc filter

that should be used in applications where slower throughput

rates are used or where higher signal-to-noise ratio or dynamic

range is desirable. The oversampling ratio of the digital filter is

controlled using the oversampling pins, OS [2:0] (see Table 8).

OS 2 is the MSB control bit, and OS 0 is the LSB control bit.

Table 8 provides the oversampling bit decoding to select the

different oversample rates. The OS pins are latched on the falling

edge of BUSY. This sets the oversampling rate for the next

conversion (see Figure 45). In addition to the oversampling

function, the output result is decimated to 18-bit resolution.

If the OS pins are set to select an OS ratio of 8, the next

CONVST x rising edge takes the first sample for each channel,

and the remaining seven samples for all channels are taken with

an internally generated sampling signal. These samples are then

averaged to yield an improvement in SNR performance. Table 8

shows typical SNR performance for both the ±10 V and the

±5 V range. As Table 8 indicates, there is an improvement in

SNR as the OS ratio increases. As the OS ratio increases, the

3 dB frequency is reduced, and the allowed sampling frequency

is also reduced. In an application where the required sampling

frequency is 10 kSPS, an OS ratio of up to 16 can be used. In

this case, the application sees an improvement in SNR, but the

input 3 dB bandwidth is limited to ~6 kHz.

The CONVST A and CONVST B pins must be tied/driven

together when oversampling is turned on. When the over-

sampling function is turned on, the BUSY high time for the

conversion process extends. The actual BUSY high time

depends on the oversampling rate selected: the higher

the oversampling rate, the longer the BUSY high, or total

conversion time (see Table 3).

Figure 44 shows that the conversion time extends as the over-

sampling rate is increased, and the BUSY signal lengthens for the

different oversampling rates. For example, a sampling frequency

of 10 kSPS yields a cycle time of 100 µs. Figure 44 shows OS × 2

and OS × 4; for a 10 kSPS example, there is adequate cycle time to

further increase the oversampling rate and yield greater improve-

ments in SNR performance. In an application where the initial

sampling or throughput rate is at 200 kSPS, for example, and

oversampling is turned on, the throughput rate must be reduced

to accommodate the longer conversion time and to allow for the

read. To achieve the fastest throughput rate possible when over-

sampling is turned on, the read can be performed during the

BUSY high time. The falling edge of BUSY is used to update the

output data registers with the new conversion data; therefore, the

reading of conversion data should not occur on this edge.

DATA:

DB[15:0]

BUSY

CONVST A,

CONVST B

tCYCLE

tCONV

4µs

9µs

19µs

OS = 0 OS = 2 OS = 4

Figure 44. No Oversampling, Oversampling × 2, and Oversampling × 4 While

Using Read After Conversion

CONVST A,

CONVST B

BUSY

OS x

tOS_SETUP

tOS_HOLD

CONVERSION N

CONVERSION N + 1

OVERSAMPLE RATE

LATCHED FOR CONVERSION N + 1

Figure 45. OS Pin Timing

Table 8. Oversample Bit Decoding

OS [2:0]

OS Ratio

SNR ±5 V

Range (dB)1

SNR ±10 V

Range (dB)1

3 dB BW ±5 V

Range (kHz)

3 dB BW ±10 V

Range (kHz)

Maximum Throughput

CONVST x Frequency (kHz)

000

No OS

90.5

91.2

200

001

92.5

93.4

100

010

94.45

95.7

13.7

18.5

011

96.5

10.3

11.9

100

99.1

100.4

12.5

101

101.7

102.8

6.25

110

103

103.5

1.5

3.125

111

Invalid

SNR values taken with a full scale 100 Hz input signal.

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