Hello,
I am using the AFE5832 and trying to understand how to the time gain control works with the up/down ramp.
My general question is, how does the engine decide the combination of attenuation and AFE gain to achieve the specified start gain / stop gain and in between?
For example if the start gain is 30dB, is the attenuator gain set to -8 and the AFE gain to 38? Or -4 + 34? Or 0 + 30? How is this specified?
Thank you!
Bertrand
Hi Bertrand,
By default, in all the TGC modes, attenuator steps are exhausted first and then LNA steps are executed.
In the up/down ramp mode, there are 160 steps. So, the first 8dB steps indicate attenuator gain (ie. 0 to 31 steps in Start_gain or Stop_gain). If both Start_gain and Stop_gain are above 31, then attenuator is always in 0-dB attenuation mode.
Thanks,
Karthik
Hi Bertrand,
Can you elaborate on the noise tradeoff that you are mentioning. What is your application and how does attenuation from LNA/attenuator affect you?
There is indeed a noise-gain tradeoff which is highlighted in Figure 9 (Input-Referred Noise vs Gain Code). In Ultrasound/NDT applications, lower gain is applied when examining objects closer to transducer (or near-field). But near-field regions also emit higher echoes. So, a higher noise in the lower gains is acceptable.
Stop gain and Start gain can both be anywhere from 0 to 159. Which means attenuation can be engaged with them.
Example 1, Start gain = 10, Stop gain = 20. Only attenuator gain is changing in this sequence. And, total gain will change from 8.5dB to 11dB (High power mode) as per Figure 2 in datasheet.
Example 2, Start gain = 40 , Stop gain = 140. Only LNA gain is changing in this sequence. Attenuator gain = 0.
By engaged, do you want to keep the attenuator fixed at fixed attenuation such as attenuator-gain -> -4dB, LNA-gain -> changes from say 20-dB to 40-dB? This is possible by using FIX_ATTEN_x = 1, ATTENUATION_x = 32.
Thanks,
Karthik
