Analog Devices ADRV9001 User Manual page 166

Reference Manual
RECEIVER GAIN CONTROL
Besides these two fields, the external gain control sets the external gain such as the gain of an LNA. Configure the LNA gain by configuring
the data structure adi_ADRV9001_RxLnaConfig_t and then calling the API adi_adrv9001_Rx_ExternalLna_Configure(). Based on the user
configuration, the ADRV9001 creates a new receiver gain table internally with extended gain indices to accommodate the additional LNA
attenuation. In such a case, the minimum possible gain index becomes 137, which provides a maximum total of 59 dB attenuation, including the
maximum LNA attenuation of 29 dB. Note that with the extended gain table, the front-end attenuation included is only 30 dB to avoid gain step
size inaccuracy from gain index 194 to 187.
As indicated in the default gain table, the external gain control uses a two bit control word through two AGPIO pins, which yields four different
gain step sizes for each receive channel. The four step sizes are based on the attenuation relative to the max gain of LNA, and are defined as
the following:
Step 0 (control word 0) = 0 dB
step 1 (control word 1) = -N dB
Step 2 (control word 2) = -N - M dB (optional)
step 3 (control word 3) = -N - M - L dB (optional)
Gain steps N, M, and L are multiple integers of 0.5 dB steps. In addition, N + M + L should not exceed 29 dB. Note that the N and M are
optional. The gain table maintains a gain step of 0.5 dB between adjacent gain indices, and it assumes that the LNA step sizes are accurate.
Create the new receiver gain table first assuming the max LNA gain (0 dB) until the ADRV9001 front-end attenuator "runs out of" attenuation.
Then new gain indices are produced by assuming the LNA gain of -N dB. To achieve the desired total attenuation by maintaining the 0.5 dB
step size, recalculate the front end gain and set it properly in the new rows. Once the front-end attenuator "runs out of" attenuation again with
LNA gain of -N dB, the new gain indices are further produced by assuming LNA gain of -N-M dB if LNA step 2 is configured. The same method
applies to LNA step 3, if it is configured.
As an example, if the LNA is configured with step 1 of 10 dB attenuation only, the receiver gain table uses LNA with 0 dB attenuation for gain
indices from 255 to 195 and sets the external gain control word to be 0. For gain entries below 195, it switches LNA 10 dB of attenuation as the
gain index 195 has an attenuation of 30 dB to maintain 0.5 dB step size. The next gain index 194 represents 30.5 dB of total attenuation with a
10 dB external LNA. Therefore, the front-end attenuation is 30.5 - 10 = 20.5 dB, which reuses the setting associated with the index 255 – 20.5
× 2 = 214. So the entries for new gain index 194 are copied from index 214, plus the external control indicates that the LNA is enabled with 10
dB attenuation step by using control word 1. Each lower gain entry is simply copied from the next lower gain in table entries 213, 212, and until
it reaches 195, which exhausts the maximum front-end attenuation.
Table 68 shows the generated new receiver gain table. It has new entries from 194 to 175 to accommodate the 10 dB LNA attenuation, and
they are copied from 214 to 195 with external gain control set to be 1.
Table 68. New Rx Gain Table Created from the Default Rx Gain Correction Table
Gain Table Front-End Attenuator Control
Index Word [7:0]
175 248 (copied from 195)
... ...
193 233 (copied from 213)
194 232 (copied from 214)
195 248
196 247
197 247
... ...
253 28
254 14
255 0
The above example uses a receiver gain correction table. The similar algorithm applies to generate the new receiver gain compensation table
with only one difference: the digital gain also compensates the LNA attenuation besides the front end attenuation. Therefore, based on the
external gain control word setting, the digital gain is further adjusted.
analog.com
TIA Control ADC Control External Gain Control
[1:0]
0 0 1
... ... ...
0 0 1
0 0 1
0 0 0
0 0 0
0 0 0
... ... ...
0 0 0
0 0 0
0 0 0
ADRV9001
Phase Offset Digital Gain/Attenuator Control Word
[10:0]
0 -2
... ...
0 -20
0 -17
0 -2
0 -17
0 −7
... ...
0 −2
0 −1
0 0
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