MSP430FR6047: MUX and USS_PGA impedance

M.A.M

Part Number: MSP430FR6047

Hi,

I am helping my customer to minimize delays and need some understanding from you on the impedance seen from outside into CH0_IN when the signal arrives (RxBias has been de-asserted).

I can see that the regular ADC12 has a 1Kohm MUX series resistance (<2V) and an input hold capacitance of 10-15pF.

But for the USS the input is a MUX followed by the PGA, so it should be possible to get some ballpark figures on an estimated impedance seen from outside?

Thanks in advance.

B.r M.A.M

over 3 years ago

0 Leonardo Estevez over 3 years ago

TI__Genius 12615 points

Hi M.A.M,

The input impedance should be given by Table 5-39. USS PHY Output Stage of the datasheet (https://www.ti.com/lit/gpn/msp430fr6047)

Please note that there are also 2 series resistors: R16 and R17 that will change the impedance seen by the transducers.

BR,
Leo

0 M.A.M over 3 years ago in reply to Leonardo Estevez

TI__Expert 8455 points

Leo,

I am afraid that I do not understand your point with Table 5-39 . I ask about the impedance of the pin CH0_IN during reception...

Was I unclear..?

B.r M.A.M

0 M.A.M over 3 years ago in reply to M.A.M

TI__Expert 8455 points

Leo,

See these two cases when input impedances matter. Please advise.

B.r, M.A.M

0 Leonardo Estevez over 3 years ago in reply to M.A.M

TI__Genius 12615 points

Hi M.A.M.,

I'm consulting with our chip designers since this isn't something we've published. Most customers have Rx Bias enabled.

BR,
Leo

0 M.A.M over 3 years ago in reply to Leonardo Estevez

TI__Expert 8455 points

Leo,

Opening the bias switches at the moment of transmission/reception is suggested here:

as well as in the "Figure21-15.Excitation" and "Figure21-17.Reception" (pages 508,510) of the Family Users Guide .

A clarification would be good. I assume that the ASQ has a pre-programmed behavior in this regard?

Thanks.

Br, M.A.M

0 Leonardo Estevez over 3 years ago in reply to M.A.M

TI__Genius 12615 points

Hi M.A.M.,

Yes - the ASQ is used in a pre-programmed configuration when an external AFE is not used. When an external AFE is used, most of the control is done in software(see ussSwLibMeasurement.c in the example project). The example project uses predefines: __AFE_EXT_3v3__, USSSWLIB_ENABLE_EXTERNAL_CIRCUITRY_CONTROL, and __AFE_EXT_5v0__ to determine whether to use external circuit control versus internal control. Channel 0 and Channel 1 are both used with the internal mux for internal control - while only Channel 0 is used when external control is used. The muxxing is done externally in this case and GPIOs are used to control the muxxing, tx power, op amps, etc.

BR,
Leo

0 M.A.M over 3 years ago in reply to Leonardo Estevez

TI__Expert 8455 points

Leo,

So for internal control, using ASQ as sequencer, how does the TxBias and RxBias switches operate in detail?

Meaning; are they open or closed when the active signal pulses pass by in the signal path (during phases Excitation and Reception)?

Thanks for clarifying.

B.r M.A.M

0 Leonardo Estevez over 3 years ago in reply to M.A.M

TI__Genius 12615 points

Hi M.A.M,

I think you can see from Figure 21-19 in the family user's guide that Rx Bias is disabled during the reception of the signal. I believe the primary contributor to the input impedance is the PGA.

BR,
Leo

0 M.A.M over 3 years ago in reply to Leonardo Estevez

TI__Expert 8455 points

Leo,

Yes I thought so.

- What is the PGA input impedance, please?

Br M.A.M

0 Leonardo Estevez over 3 years ago in reply to M.A.M

TI__Genius 12615 points

Hi M.A.M.,

I've sent you a message on this.

BR,

Leo

0 M.A.M over 3 years ago in reply to Leonardo Estevez

TI__Expert 8455 points

Thanks Leo.

Then we are OK.

Cheers.

B.r M.A.M

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MSP430FR6047: MUX and USS_PGA impedance