PGA460-Q1: BPF coefficients in PGA460-Q1

Part Number: PGA460-Q1
Other Parts Discussed in Thread: PGA460

Hello.

I would like to calculate BPF coefficients to use unique BPF in PGA460-Q1.

Can you tell me about the following questions?

I calculated BPF coefficients A2, A3 and B1 from the filter center frequency and the bandwidth with refference to this page.

[Resolved] PGA460: Frequency offset in high frequency mode - Sensors forum - Sensors - TI E2E support forums

However, the register value in PGA-460 and the result using the formula in the page did not match completely.

For example, if Bandwidth is 8kHz,

A3 = -399.3*Bandwidth+65512 = 1609.6

However, The register value was 1607(register value : 0x0647).

Is this difference due to the digit loss?

And, I presume A2, B1 are signed 16-bit, A3 is an unsigned 16bit. Is that correct?

Is the BPF filter(second-order Butterworth IIR type) used in PGA460-Q1 the following transfer function?

( a2, a3 and b1 correspond to BPF coefficients A2, A3 and B1 respectively.)

over 4 years ago

0 Isaac Lara over 4 years ago

TI__Mastermind 23045 points

Hello tr,

1. To my knowledge A2, A3, and B1 registers are all 16-bit signed registers, but as far as the values not matching in the registers it seems like the issue is that the coefficients A3 and B1 calculated and are updated depending automatically depending on the value entered for the filter bandwidth and burst parameter. A2 is the only parameter that needs to calculated and entered. You can read about the details here: https://e2e.ti.com/support/sensors/f/sensors-forum/838216/pga460-calculate-bpf-and-lpf-coeficients

2. The transfer function for the second-order Butterworth IIR BP filter used is not included in the datasheet due to the proprietary nature of this integrated DSP.

Best,

Isaac

0 tr over 4 years ago in reply to Isaac Lara

Prodigy 220 points

Hi Isaac,

1. I recognized A2 is the parameter that needs to enter only if use in high-frequency range mode.
Do I need to enter A2 coefficient to use in low-frequency range mode?

Also about low-pass filter coefficient A2,

if co=4kHz

A2=202.7*co-32765=202.7*4-32765=-31,954.2(the formula in your link)

However, LPF_A2 register value is 31954(0x7CD2)

Why are the signs of these values inverted?

2. I'm sorry to know the transfer function.However, thank you for your help.

0 Isaac Lara over 4 years ago in reply to tr

TI__Mastermind 23045 points

Hello tr,

You read that correct, the value for A2 only needs to be manually entered if the device is being operated in High Frequency range so anything 180-480kHz.

For the low-pass filter coefficient if you look at the data sheet for the PGA460 in specific the LPF_A2_MSB register, the register map shows the MSB is a reserved bit in that register that takes into consideration the negative sign for the user.

Best,

Isaac