AD7730/AD7730L

–10–

OUTPUT NOISE AND RESOLUTION SPECIFICATION

The AD7730 can be programmed to operate in either chop mode or nonchop mode. The chop mode can be enabled in ac-excited or

dc-excited applications; it is optional in dc-excited applications, but chop mode must be enabled in ac-excited applications. These

options are discussed in more detail in later sections. The chop mode has the advantage of lower drift numbers and better noise im-

munity, but the noise is approximately 20% higher for a given –3 dB frequency and output data rate. It is envisaged that the majority

of weigh-scale users of the AD7730 will operate the part in chop mode to avail themselves of the excellent drift performance and

noise immunity when chopping is enabled. The following tables outline the noise performance of the part in both chop and nonchop

modes over all input ranges for a selection of output rates. Settling time refers to the time taken to get an output that is 100% settled

to new value.

Output Noise (CHP = 1)

This mode is the primary mode of operation of the device. Table I shows the output rms noise for some typical output update rates

and –3 dB frequencies for the AD7730 when used in chopping mode (CHP of Filter Register = 1) with a master clock frequency of

4.9152 MHz. These numbers are typical and are generated at a differential analog input voltage of 0 V. The output update rate is

selected via the SF0 to SF11 bits of the Filter Register. Table II, meanwhile, shows the output peak-to-peak resolution in counts for

the same output update rates. The numbers in brackets are the effective peak-to-peak resolution in bits (rounded to the nearest 0.5

LSB). It is important to note that the numbers in Table II represent the resolution for which there will be no code flicker within a

six-sigma limit. They are not calculated based on rms noise, but on peak-to-peak noise.

The numbers are generated for the bipolar input ranges. When the part is operated in unipolar mode, the output noise will be the

same as the equivalent bipolar input range. As a result, the numbers in Table I will remain the same for unipolar ranges while the

numbers in Table II will change. To calculate the numbers for Table II for unipolar input ranges simply divide the peak-to-peak

resolution number in counts by two or subtract one from the peak-to-peak resolution number in bits.

Table I. Output Noise vs. Input Range and Update Rate (CHP = 1)

Typical Output RMS Noise in nV

Output

–3 dB

SF

Settling Time

Settling Time

Input Range

Input Range

Input Range

Input Range

Data Rate Frequency

Word

Normal Mode

Fast Mode

= 80 mV

= 40 mV

= 20 mV

= 10 mV

50 Hz

1.97 Hz

2048

460 ms

60 ms

115

75

55

40

100 Hz

3.95 Hz

1024

230 ms

30 ms

155

105

75

60

150 Hz

5.92 Hz

683

153 ms

20 ms

200

135

95

70

200 Hz*

7.9 Hz

512

115 ms

15 ms

225

145

100

80

400 Hz

15.8 Hz

256

57.5 ms

7.5 ms

335

225

160

110

*Power-On Default

Table II. Peak-to-Peak Resolution vs. Input Range and Update Rate (CHP = 1)

Peak-to-Peak Resolution in Counts (Bits)

Output

–3 dB

SF

Settling Time

Settling Time

Input Range

Input Range

Input Range

Input Range

Data Rate Frequency

Word

Normal Mode

Fast Mode

= 80 mV

= 40 mV

= 20 mV

= 10 mV

50 Hz

1.97 Hz

2048

460 ms

60 ms

230k (18)

175k (17.5)

120k (17)

80k (16.5)

100 Hz

3.95 Hz

1024

230 ms

30 ms

170k (17.5)

125k (17)

90k (16.5)

55k (16)

150 Hz

5.92 Hz

683

153 ms

20 ms

130k (17)

100k (16.5)

70k (16)

45k (15.5)

200 Hz*

7.9 Hz

512

115 ms

15 ms

120k (17)

90k (16.5)

65k (16)

40k (15.5)

400 Hz

15.8 Hz

256

57.5 ms

7.5 ms

80k (16.5)

55k (16)

40k (15.5)

30k (15)

*Power-On Default

Output Noise (CHP = 0)

Table III shows the output rms noise for some typical output update rates and –3 dB frequencies for the AD7730 when used in non-

chopping mode (CHP of Filter Register = 0) with a master clock frequency of 4.9152 MHz. These numbers are typical and are gen-

erated at a differential analog input voltage of 0 V. The output update rate is selected via the SF0 to SF11 bits of the Filter Register.

Table IV, meanwhile, shows the output peak-to-peak resolution in counts for the same output update rates. The numbers in brackets

are the effective peak-to-peak resolution in bits (rounded to the nearest 0.5 LSB). It is important to note that the numbers in Table

IV represent the resolution for which there will be no code flicker within a six-sigma limit. They are not calculated based on rms

noise, but on peak-to-peak noise.

The numbers are generated for the bipolar input ranges. When the part is operated in unipolar mode, the output noise will be the

same as the equivalent bipolar input range. As a result, the numbers in Table III will remain the same for unipolar ranges while the

numbers in Table IV will change. To calculate the number for Table IV for unipolar input ranges simply divide the peak-to-peak

resolution number in counts by two or subtract one from the peak-to-peak resolution number in bits.

REV. B