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VCA2615: The difference of control gain amplitude between VGA chips exceeds the specification limit

Part Number: VCA2615

Hello!
We have some questions when using vca2615rgzt. I hope you can give me some help and answers.
Problem: we use vca2615 as part of automatic gain control. Under the condition of almost the same input and output, the maximum difference of VCNTL control voltage of different vca2615 is 0.2V. According to the corresponding curve relationship (as shown in Figure 1), the VCNTL difference of 0.2V means that there is a difference of 5dB magnification. However, the gain error in the chip manual is - 1.5dB and + 1.5dB (as shown in Figure 2).
I would like to know if the 5dB difference in gain between vca2615 devices is normal.
The schematic diagram of our vca2615 part is shown in Figure 3.
Main configuration of vca2615:
LNP Gain(G1,G2)=11;
FB(1-4)=1111;

Vcainsel - used to time-sharing switch the signals of ChannelA and ChannelB to VCAoutA and complete sampling.
Relevant tests we conducted:
Input signal: 3.7mhz sine wave (input from a and B in Fig. 3)
Amplitude: less than 1mV (stable)。
We use an oscilloscope to measure its waveform, through FFT test to the corresponding waveform amplitude corresponding DB value. DB value of signal at each test point. Through their differences, it is found that the VGA part of the chip is the main error source. I'm not sure the VGA part of this error is reasonable.
Table 1 is the data of our test quantity (converted to DB value through FFT function of oscilloscope)

The related schematic diagram and test data are in the attachment word.(I can't upload my schematic and data in this frame

thank you!VCA2615 chip control gain difference exceeds specification limit.doc

  • Hello,

    Due to the Thanksgiving holidays, our responses are delayed.

    We will get back to you by early next week.

  • Hi, 

    Thanks for the detail measurements!

    let me try to explain the VCA operaiton. it is not AGC type chip. it is voltage controlled amplifier. So AGC operation may not fit. 

    it is true the figure 1 includes the gain settings for the whole channel. it is roughly 5dB per 0.2V.  when you measure the VCA only path, the 5dB per 0.2V number may vary. however, i don't expect much change. in your data, the theoritcal  calculation is 4.7dB, while actual measurement is 1.45dB. 

    I suspect that your measurement may be impacted by error due to 1mVpp small signal. when the Vcntl=1V, the gain of the chip is 10dB to 30dB depending on setting. So i think you can use 50mVpp signal to measure for minmizing the noise impact.  

    the gain error is limit accross chips. +/-1.5dB error to the ideal linear curuve. TI did trimming before shippment. 

    Thanks!