AD6654BBC datasheet(51/88 Pages) AD

51 / 88 page

AD6654

Rev. 0 | Page 51 of 88

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

representation, 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 Gain Value 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

the 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 the

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

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

expression for the AGC loop is given by

()

)

(

)

(

)

(

−

GCLOSED

Program K1 and K2 (the gain parameters) and Pole P through

AGC loop Gain 1 and Gain 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 and 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.

The relationship between the time constant and the closed-loop

poles that can be used for this purpose is

⎥

⎦

⎤

⎢

⎣

⎡

CIC

1,2

Rate

Sample

exp

where τ1, 2 are the time constants corresponding to Pole P1 and

Pole P 2

The time constants can also be derived from settling times as

given by

time

settling

time

settling

MCIC (CIC decimation is from 1 to 4,096), and either the settling

time or time constant are chosen by the user. The sample rate is

the sample rate of the stream coming into the AGC. If channels

were interleaved in the output data router, then the combined

sample rate into the AGC should be considered. This rate

should be used in the calculation of poles in the previous

equation, where the sample rate is mentioned.

The loop filter output corresponds to the signal gain that is

updated by the AGC. Because all computation in the loop filter

is done in logarithmic domain (to the Base 2) of the samples,

the signal gain is generated using the exponent (power of 2) of

the loop filter output.

The gain multiplier gives the product of the signal gain with

both the I and Q data entering the AGC section. This signal

gain is applied as a coarse 4-bit scaling and then as a fine scale

8-bit multiplier. Therefore, the applied signal gain is from 0 to

96.3 dB in steps of 0.024 dB. The initial signal gain is program-

mable using the AGC signal gain register. This register is again a

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