AFE5808

Chandu,

The VCAT is the analog TGC, a DC or AC input will control the envelope of signal.  If your question is why an attenuator instead of a gain stage, this is done because of better DC offset control between gain stages.

Thanks,

Chuck Smyth

Hi Chandu,

After the TX pulse you start the RX mode acquisition. At the begining of the acquisition you have a strong echoes from near located objects and in the time domain this echoes are weaker and weaker because the acoustic signal attenuates exponentially with depth. In order to equalize amplification over the penetration depth (time), you need to attenuate the signal more in the near field and leave it less attenuated in the far field (Time_Gain_Control). Hope it's clear.

Regards..

Chandu,

Mariusz's answer is correct.  I wasn't sure how deep your question was.  The function of the Attenuator is for Analog TGC to quickly and accurately vary the Gain with variable envelopes.

Thanks,

Chuck Smyth

Hi, Thank for your information on VCAT. Similarly I just want to know the purpose, working of the blocks of AFE5808(LNA,VCAT,PGA,LPF) , Can provide document or its source, which can specify about the processing of received ultra sound signals.

  I want understand purpose of VCNTL pin for VCAT, which I could not understand from datasheet, can you give me a example application of the pin. Some circuit is given for VCNTL on-board generation circuit in AFE Evm schematic, what is the working of the circuit.

thank you.

regards,

chandu

Chandu,

The purpose of the VCNTL pins are for Time-Gain Compensation (TGC), where the gain of the VCAT block is varied over time.  This can be used in the ultrasound system to offset natural attenuation at depth of field.  The VCNTL is an analog gain control used to vary the gain in time.  You can ramp the DC voltage on these pins to vary the attenuation.

Thanks,

Chuck Smyth

For medical diagnosis to get Time gain compensation, Can you suggest me VCNTL analog signal. I think it is ramp, but I can't understand what should be the frequency. On what criteria the analog signal is determined for TGC.

Thank you.

chandu.

Hi Chandu,

It's very easy. From elementary physics you should know that V=S/t (V-velocity, S-distance, t-time). Of course we consider a constant velocity in this case. An object formula is V=dS/dt. In medical applications we assume a mean sound velocity is constant and its average value is about 1560m/s.

However, in medical applications, the first formula must be modified: V=2*S/t. Why? Just because the sound travels from a transducer to the object, reflects and travels the same distance back to the transducer. That's why the distance should be doubled.

Example:

Suppose you want to have penetration depth 20cm. How much time does take the acquisition?

V=2*S/t => t=2*S/V , let's calculate: t= 2*0.2/1560 = 2.6ms

This is a time duration of a TGC ramp signal.  How frequently should we repeat this signal?

f=1/t  (1/2.6ms=3.9kHz)

It's WRONG to say TGC signal frequency. Correct should be "TGC signal repetition frequency".

Hope it's clear.

Regards,

Mariusz

To all TI employees : I STRONGLY advice to make a small correction in all ultrasound AFE's datasheets. For experts it's clear, however for novices it could be confusing. "TGC frequency" should be modified into "TGC repetition frequency".

Regards,

Mariusz

Mariusz, 

thanks for answering the quesitons! i agree with you that TGC frequency is not a common term. Our AFE5808/9 datasheets use pulse repetition frequency (PRF) since it is a more common term in ultrasound industry.

Just i would like to comment on the 2.6ms number in the previous post. it should be 260us for 20cm depth. ms is very uncommon in medical applications.  for big animal imaging systems or sonar systems, ms TGC control can exist. 

in our AFE5808A/9 datasheets, we recommended the following. it may be helpful for the VCntl circuit design. 

Thanks!

Typical VCNTLM,P signals are generated by an 8bit to 12bit 10MSPS digital to analog converter (DAC) and a

differential operation amplifier. TI’s DACs, such as TLV5626 and DAC7821/11 (10MSPS/12bit), could be used to
generate TGC control waveforms. Differential amplifiers with output common mode voltage control (that is,
THS4130 and OPA1632) can connect the DAC to the VCNTLM/P pins. The buffer amplifier can also be configured
as an active filter to suppress low frequency noise. The VCNTLM/P circuit shall achieve low noise in order to
prevent the VCNTLM/P noise being modulated to RF signals. It is recommended that VCNTLM/P noise is below 25nV/rtHz at 1KHz and 5 nV/rtHz at 50 KHz. More information can be found in the literatures SLOS318F and
SBAA150. The VCNTL vs Gain curves can be found in Figure 3. The below table also shows the absolute gain vs.
VCNTL, which may help program DAC correspondingly.

Hi Xiaochen,

The PRF acronym is very well known in ultrasound industry, however it is related with the TX "Fire" signal. Of course, the analogue TGC control signal MUST be repeated with the same frequency. For you and for me it is clear, however for novices "TGC frequency" could be confusing. That's why I suggest such a small correction in TI AFE's datasheets.

2.6ms !!  Oooopss....  Shame on me!!  Back to Grammar School :))

Best Regards,

Mariusz

Mariusz,

Where have you seen the term "TGC Frequency"?  I can't find this in our latter datasheets.

Thanks,

Chuck Smyth

Hi Chuck,

Well, I also can't find :)))  According to Chandu's question I have simply assumed that such a term must be in the datasheet. Sorry for false alarm.

Best Regards,

Mariusz