Product Preview
WM8777
MAXIMUM GAIN (ALC ONLY) AND MAXIMUM ATTENUATION
To prevent low level signals being amplified too much by the ALC, the MAXGAIN register sets the
upper limit for the gain. This prevents low level noise being over-amplified. The MAXGAIN register
has no effect on the limiter operation.
The MAXATTEN register has different operation for the limiter and for the ALC. For the limiter it
defines the maximum attenuation below the static (user programmed) gain. For the ALC, it defines
the lower limit for the gain.
REGISTER
ADDRESS
BIT
LABEL
DEFAULT
DESCRIPTION
(1Dh)
6:4
MAXGAIN[2:0]
111
(+24dB)
Set maximum gain for the PGA (ALC
only)
ALC Control 1
111 = +24dB
110 = +20dB
…..(-4dB steps)
010 = +4dB
001 = 0dB
000 = 0dB
(21h)
3:0
MAXATTEN
[3:0]
0110
Maximum attenuation of PGA
Limiter Control
Limiter
(attenuation
below static)
ALC (lower PGA
gain limit)
1010 or lower
= -1dB
0000 = -3dB
0001 = -4dB
0010 = -5dB
…. (-1dB steps)
1001 = -12dB
1011 = -5dB
….. (-4dB steps)
1110 = -17dB
1111 = -21dB
Table 77 ALC MAXGAIN and MAXATTEN Registers
HOLD TIME (ALC ONLY)
The ALC also has a hold time, which is the time delay between the peak level detected being below
target and the PGA gain beginning to ramp up. It can be programmed in power-of-two (2n) steps, e.g.
2.67ms, 5.33ms, 10.67ms etc. up to 43.7ms. Alternatively, the hold time can also be set to zero. The
hold time only applies to gain ramp-up, there is no delay before ramping the gain down when the
signal level is above target.
REGISTER
ADDRESS
BIT
LABEL
DEFAULT
DESCRIPTION
(1Eh)
3:0
HLD[3:0]
0000
ALC hold time before gain is
increased.
ALC Control 2
0000 = 0ms
0001 = 2.67ms
0010 = 5.33ms
… (time doubles with every step)
1111 = 43.691s
Table 78 ALC Hold Time Register
OVERLOAD DETECTOR (ALC ONLY)
To prevent clipping when a large signal occurs just after a period of quiet, the ALC circuit includes an
overload detector. If the ADC input signal exceeds 87.5% of full scale (–1.16dB), the PGA gain is
ramped down at the maximum attack rate (as when ATK = 0000), until the signal level falls below
87.5% of full scale. This function is automatically enabled whenever the ALC is enabled.
(Note: If ATK = 0000, then the overload detector makes no difference to the operation of the ALC. It
is designed to prevent clipping when long attack times are used).
PP Rev 1.94 November 2004
71
w
WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
