AD9874ABST datasheet(30/40 Pages) AD

30 / 40 page

REV. A

AD9874

–30–

the maximum bandwidth is 9 kHz. A general expression for the

attack bandwidth is:

MHz

CLK

AGCA

=×

() × ()

(8)

and the corresponding attack time is:

attack

AGCA

=× ×







 =

()

100

0 35

(9)

assuming that the loop dynamics are essentially those of a

single-pole system.

The 4-bit code in the AGCD field sets the ratio of the attack

time to the decay time in the amplitude estimation circuitry.

When AGCD is zero, this ratio is one. Incrementing AGCD

multiplies the decay time constant by 2

1/2, allowing a 180:1

range in the decay time relative to the attack time. The decay

time may be computed from:

decay

attack

AGCD

=× ()

(10)

Figure 21a shows the AGC response to a 30 Hz pulse-modu-

lated IF burst for different AGCA and AGCD settings.

AGCA = 0

TIME – ms

AGCA = 4

AGCA = 8

AGCD = 8

AGCD = 0

AGCD = 8

AGCD = 0

AGCD = 8

AGCD = 0

Figure 21a. AGC Response for Different AGCA

and AGCD Settings with fCLK = 18 MSPS,

fCLKOUT = 20 kSPS, Decimate by 900, and AGCO = 0

The 3-bit value in the AGCO field determines the amount of

attenuation added in response to a reset event in the ADC.

Each increment in AGCO doubles the weighting factor. At the

highest AGCO setting, the attenuation will change from 0 dB to

12 dB in approximately 10

µs, while at the lowest setting the

attenuation will change from 0 dB to 12 dB in approximately

1.2 ms. Both times assume fCLK = 18 MHz. Figure 21b shows

the AGC attack time response for different AGCO settings.

TIME – ms

0.1

0.2

0.5

0.4

0.3

1.0

0.9

0.8

0.7

0.6

112

128

AGCO = 7

AGCO = 4

AGCD = 0

Figure 21b. AGC Response for Different AGCO

Settings with fCLK = 18 MSPS, fCLKOUT = 300 kSPS,

Decimate by 60, and AGCA = AGCD = 0

Lastly, the AGCF bit reduces the DAC source resistance by at

least a factor of 10. This facilitates fast acquisition by lowering

the RC time constant that is formed with the external capacitors

connected from the GCP pin-to-ground (GCN pin). For an

overshoot-free step response in the AGC loop, the capacitor

connected from the GCP pin to the GCN ground pin should be

chosen so that the RC time constant is less than one quarter of

the raw loop. Specifically:

< 18

()

(11)

where R is the resistance between the GCP pin and ground

(72.5 k

30% if AGCF = 0, < 8 k

if AGCF = 1) and BW is

the raw loop bandwidth. Note that with C chosen at this upper

limit, the loop bandwidth increases by approximately 30%.

Now consider the case described above but with the DVGA

enabled to minimize the effects of 16-bit truncation. With the

DVGA enabled, a control loop based on the larger of the two

estimated signal levels (i.e., output of DEC1 and DVGA) is

used to control the DVGA gain. The DVGA multiplies the

output of the decimation filter by a factor of 1 to 4 (i.e., 0 dB to

12 dB). When signals are small, the DVGA gain is 4 and the

16-bit output is extracted from the 24-bit data produced by the

decimation filter by dropping 2 MSB and taking the next 16

bits. As signals get larger, the DVGA gain decreases to the point

where the DVGA gain is 1 and the 16-bit output data is simply

the 16 MSB of the internal 24-bit data. As signals get even

larger, attenuation is accomplished by the normal method of

increasing the ADC’s full scale.

The extra 12 dB of gain range provided by the DVGA reduces

the input-referred truncation noise by 12 dB and makes the data

more tolerant of LSB corruption within the DSP. The price

paid for this extension to the gain range is that the start of AGC

action is 12 dB lower and that the AGC loop will be unstable if its

bandwidth is set too wide. The latter difficulty results from the

large delay of the decimation filters, DEC2 and DEC3, when

one implements a large decimation factor. As a result, given an

option, the use of 24-bit data is preferable to using the DVGA.

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