AGC with AD8338

Question

I am trying to design an AGC with AD8338 along with AD704 as an Transimpedance amplifier.
I need the details on :
1) Attack time equation for AD8338 given in datasheet.
2) The connection details on connection for AD8338 as an AGC.
3) How to set threshold values for output and control the output based on threshold values set.
4) How to control Dynamic Gain range of the AGC.

Hooman · Accepted Answer

Hi rookie1, 
 Yes, the AD8338 input referred noise voltage (G = 0dB) is 150nV/RtHz (a large number compared to generic op amps in the 10's of nV/RtHz range worst case) which would result in a rather large NF. 
 Here is what I calculate for NF assuming a 50ohm source impedance (0.91nV/RtHz noise density): 
 From the datasheet, with Vg = 0.1V --> Vout_noise = 1000nV/RtHz 
 NF = 10* log [1 + Na / (Ni * G)] = 10* log [ 1 + (1000nV/RtHz)^2 / ((0.91nV/Hz)^2 *1) ] = 60.8dB 
 Keep in mind that if you're dialing down the gain to the minimum 0dB, then you're dealing with a fairly large input signal to begin with. So, with such a large input signal, noise is not going to be a significant factor in most applications. The NF impact becomes a larger issue as gain is increased. At least, that's how I look at it. 
 Another way to look at this, with low AD8331 gain setting, your input SNR is already huge. So, the large NF of AD8331 would not impact anything. For example, assume a 1V_RMS input signal over a 10MHz bandwidth and 50ohm source impedance: 
 Input_SNR = 20 * log (1V_RMS / (0.91nV/RtHz * sqrt(10e6)) ) = 110.8dB 
 At the AD8338 output, the SNR will be: 
 Output_SNR = 110.8dB - 60.8dB = 50dB 
 
 Regards, 
 Hooman)

Hooman · Answer

Hi rookie1, 
 Does this older post answer your question about the noise factor of AD8338? 
 https://ez.analog.com/amplifiers/f/q-a/539393/ad8338 
 
 Regards, 
 Hooman

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AGC with AD8338