AD6636PCB datasheet(41/72 Pages) AD

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AD6636

Rev. 0 | Page 41 of 72

operation in the logarithmic domain. The rms samples obtained

are subtracted from the request signal level R specified in the

AGC desired level register, leaving an error term to be processed

by the loop filter, G(z).

The user sets this programmable request signal level R accord-

ing to the output signal level that is desired. The request signal

level R is programmable from −0 dB to −23.99 dB in steps of

0.094 dB.

The request signal level should also compensate for errors, if

any, due to the CIC scaling, as explained previously in this

section. Therefore, the request signal level is offset by the

amount of error induced in CIC, given by

Offset = 10 × log(MCIC × Navg) − SCIC × 3.01 dB

where Offset is in dB.

Continuing the previous example, this offset is given by

Offset = 72.24 − 69.54 = 2.7 dB

So the request signal level is given by

dBFS

094

)

(

⎥⎦

⎤

⎢⎣

⎡

−

Offset

DSL

ceil

where:

R is the request signal level.

DSL (desired signal level) is the output signal level that the user

desires.

Therefore, in the previous example, if the desired signal level is

−13.8 dB, the request level R is programmed to be −16.54 dB,

compensating for the offset.

This request signal level is programmed in the 8-bit AGC

desired level register. This register has a floating-point represen-

tation, where the 2 MSBs are exponent bits and the 6 LSBs are

mantissa bits. The exponent is in steps of 6.02 dB, and the

mantissa is in steps of 0.094 dB. For example, a value 10’100101

represents 2 × 6.02 + 37 × 0.094 = 15.518 dB.

The AGC provides a programmable second-order loop filter.

The programmable parameters gain 1 (K1), gain 2 (K2), error

threshold E, and pole P completely define the loop filter

characteristics. The error term after subtracting the request

signal level is processed by the loop filter, G(z). The open loop

poles of the second-order loop filter are 1 and P, respectively.

The loop filter parameters, pole P and gain K, allow the

adjustment of the filter time constant that determines the

window for calculating the peak-to-average ratio.

Depending on the value of the error term that is obtained after

subtracting the request signal level from the actual signal level,

either gain value, K1 or K2, is used. If the error is less than the

programmable threshold E, K1 or K2 is used. This allows a fast

loop when the error term is high (large convergence steps

required) and a slower loop function when error term is smaller

(almost converged).

The open-loop gain used in the second-order loop G(z) is given

by one of the following equations:

K = K1, if Error < Error Threshold

K = K2, if Error > Error Threshold

The open-loop transfer function for the filter, including the gain

parameter, is

()

−

If the AGC is properly configured in terms of offset in request

level, then there are no gains in the AGC loop except for the

filter gain K. Under these circumstances, a closed-loop

expression for the AGC loop is given by

()

−

Gclosed

The gain parameters K1, K2, and pole P are programmable

through AGC loop gain 1, 2, and AGC pole location registers

from 0 to 0.996 in steps of 0.0039 using 8-bit representation. For

example, 1000 1001 represent (137/256 = 0.535156). The error

threshold value is programmable between 0 dB and 96.3 dB in

steps of 0.024 dB. This value is programmed in the 12-bit AGC

error threshold register, using floating-point representation. It

consists of four exponent bits and eight mantissa bits. Exponent

bits are in steps of 6.02 dB and mantissa bits are in steps of

0.024 dB. For example, 0111’10001001 represents 7 × 6.02 + 137

× 0.024 = 45.428 dB.

The user defines the open-loop pole P and gain K, which also

directly impact the placement of the closed-loop poles and filter

characteristics. These closed-loop poles, P1, P2, are the roots of

the denominator of the previous closed-loop transfer function

and are given by

)

(

)

(

−

Typically, the AGC loop performance is defined in terms of its

time constant or settling time. In this case, the closed-loop poles

should be set to meet the time constants required by the AGC

loop.

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