[PGA280 + ADS1259] Crosstalk and other questions

Mateus

Other Parts Discussed in Thread: ADS1259, PGA280

Hi !!

I'm using PGA280 + ADS1259 solution in an analog voltage/current input system. I have multiple input ports, each one with a RC filter at input stage and connected to external multiplexers. External multiplexers are connected to the PGA input ports. Between PGA and ADS1259, I'm using datasheet recommended RC filter. Voltage reference is set to internal and data rate is 14400 SPS.

Using this HW implementation, i have found some crosstalk/intereference between channels. I believe it is related to the settling time of RC filters vs data rate and channels reading. Because of it, I would like to verifiy some doubts about HW and SW implementation.

1. As I'm using external multiplexers (they are not controled by PGA mux control feature at register 0) and at each reading I have to configure PGA gain and input switch control (Register 6), what is the best steps sequence for external multiplexers and PGA configuration ?

Nowadays, I disable external mux, configure PGA gain, configure PGA switches, configure/enable external multiplexer and send start command.

2. About PGA current buffer. According to datasheet: " ... as soon as buffers are turned off, amplifiers settle back to high precision." My question is if I send a start command and buffer are still turned on, my precision will be compromissed ? To avoid it, must I use the start delay ?

To enable buffer feature, I have bit 5 ('T') in command byte. When I have to use it ? Only when doing external mux control or can I use in each PGA write command, as example, set gain, send start command ?

About HW implementation, I can send my schematics. Just give an e-mail.

Regards,

Mateus.

over 13 years ago

0 Luis Chioye over 13 years ago

TI__Guru 55466 points

Hi Mateus,

The RC filter at the input of the ADS1259 needs to completely settle before you start the conversion. If the RC filter that is used is formed with 2x47 ohm resistors and a 10nF capacitor, the corner frequency of the filter is in the order of:

f-3dB= 1/{(2π(C(R1 + R2))} = 170kHZ;

For the analog filter to completely settle the user need to wait several RC time constants. For example, when performing a 20 bit resolution measurement, after changing the channel, the user will need to wait up to 14xRC filter time constants for the measurement to settle within ½ an LSB of a 20 bit measurement or 17xRC time contants for a measurement to settle within ½ an LSB of a 24 bit measurement.

- The analog filter at the output of the PGA280 must be allowed to settle before triggering the start of the conversion. The programmable Initial START Delay may be used to avoid these settling time issues (DELAY[2:0] register). The conversion is trigered after the current buffer is disabled and the analog RC filters are settled.

When changing PGA gain, the RC PGA output filter must also be allowed to settle.

The current buffer function of the PGA280 is primarily used when switching channels on a multiplexer to avoid an over flow condition at the inputs of the PGA (due to the fast transient at the inputs of the PGA); the current buffer is activated to avoid inrush currents at the inputs. Page 20 of the PGA280 datasheet discusses these inrush currents through the input clamps of the PGA280; where this current could interact with any RC filters that may be present at the inputs of the PGA280. It is not necessary to enable the current buffers everytime that you send a START command or when you change gains. However, when changing the PGA280 gain, the output analog RC filter of the PGA280 must be allowed to settle before trigering the conversion.

Thank you and Best Regards,

Luis

0 Mateus over 13 years ago in reply to Luis Chioye

Prodigy 110 points

Hi Luis,

Thanks for your reply. Based on your comments, if I have 24 bits of resolution and 14400 SPS:

Conversion time using sync1 = 424 us

RC PGA output settling time = 8us

PGA buffer function time = this time must be equal or more than mux enable/transition time or mux enable/transition time + RC PGA input time ?

Mux enable/transition channel time = less than 1us

RC PGA input settling time = 2.2us (2x10k + 220pF)

My total time is

T = RC PGA input filter time + Mux time (= buffer time) + RC PGA output filter time + Conversion time.

So, I have these steps:

1) Set external mux channel

2) Set PGA internal switch channel

3) Set gain

4) Send start command (with delay equal to RC PGA settling time)

Is it correct ?

0 Luis Chioye over 13 years ago in reply to Mateus

TI__Guru 55466 points

Hi Mateus,

One clarification/correction from my previous post: The PGA280 datasheet recommends enabling the current buffer when changing channels of a mux and/or when changing the PGA gains where a transients/spikes may occur.

Please find suggestions below.

if you have 24 bits of resolution and 14400 SPS, Conversion time using sync1 = 424 us:

Correct, 424 us is the conversion time when using START pin to trigger the Conversion when the ADS1259 is cofigured with the SINC1 filter.

RC PGA output settling time = 8us:

If you are using a PGA output filter with two 47Ohm resistors and a 10nF differential cap, the necessary settling time to a 1/2 of LSB of a 24 bit measurement is ~17.33x RC time constants.

T output filter = (47Ohm+47Ohm)*10nF = 0.94us

Settling Time of RC at PGA output for a 24 bit measurement = 17.33*T = ~ 16.3us

PGA buffer function time = this time must be equal or more than mux enable/transition time or mux enable/transition time + RC PGA input time ?

Mux enable/transition channel time = less than 1us

RC PGA input settling time = 2.2us (2x10k + 220pF)

You will have to enable the PGA280 current buffer when switching mux channels during the time any transient is present at the inputs of the PGA. The PGA280 datasheet recommends enabling the current buffer when changing channels of a mux and/or when changing the PGA gain where a transients/spikes may occur

Since the PGA280 current buffer is only to isolate the differential amplifier for any transient that may occur, enabling the buffer for a few micro seconds is probably sufficient. If you are controlling the PGA280 buffer by software command, the buffer active time is programmed with Register 3 BUFTIM[5:0] setting. Notice that the minimum turn on time that can be programmed is 6us. You may need to experiment with the Register 3 BUFTIM[5:0] settings. Alternatively, you could activate/deactivate the PGA280 current buffer externally using GPIO4. Please refer to pages 36 and 37 of the PGA280 datasheet.

If the input filter at the PGA input is formed with two10kOhm resistors and a 220pF differential capacitor, the RC input filter time constant is:

T input filter = (10kOhm+10kOhm)*2.2pF = 4.4us

After all the mux and gain settings are performed, and after the current buffer is disabled, all the filters (input and output filters) must be allowed to settle to a 1/2 of LSB of the desired resolution... On this example, the PGA280 RC input filter time constant is 4.4us; and the PGA280 RC output filter time constant is 0.94us. The input filter has the longest time constant, therefore you will need to wait 17.33 input filter time constants (4.4us*17.3= ~76us) for both filters to completely settle.

There are different ways to accomplish the sequence. One possible sequence may be:

1) Enable PGA280 Current buffer using GPIO4 (or enable the current buffer by software command with the corresponding programmed buffer delay BUFTIM[5:0] setting).

2) Set external mux channel

4) Set PGA internal switch channel

5) Set PGA gain

6) After a delay of a few microseconds (~6us or adjust as needed), disable the PGA280 current buffer (or alternatively wait for thePGA280 programmed buffer delay).

7) After a 76us delay to allow the RC filter to settle, set the ADS1259 START pin high to trigger the conversion.-- Alternatively, you can easily program on the ADS1259 a fixed initial STARTdelay DELAY[2:0] of 69.4uS and the device will automatically wait the programmed delay to start the conversion after STARTgoes high...

8) The conversion results will be available after the 424us conversion time + the 76us delay.

Best Regards,

Luis

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[PGA280 + ADS1259] Crosstalk and other questions