ADS1158: Circuit review request

Hi Danny,

A few comments for you:

• I don't see any input filtering on the ADS1158's analog inputs. This might be necessary for anti-aliasing
• There are no load capacitors shown on the crystal. These are especially important to make sure the crystal oscillates and can actually provide a clock signal to the ADC. You can see that this is a common challenge other ADS1158 users have experienced, so take care to tune your circuit appropriately. We would recommend an external oscillator, as opposed to a crystal, for best performance.
• Looks like you are applying a 4.096V VREF to VREFP since you are using REF5040? But the VREF supply is only 2.5V? If AVDD = 2.5 V, then the voltage on VREFP cannot be greater than AVDD + 0.1V, though I am not really sure what you are trying to do here.
• We usually recommend some small series resistance on the SPI lines for best performance, usually 30-50 ohms

Otherwise, you have used the circuitry provided by the datasheet, so you should be good to go.

-Bryan

Hi Bryan

Thanks for the reply
I will explain the circuit further
I refer to page 42 (figure 54. Multichannel, + -10V single-Ended input, Bipolar Supply Operation) of the ADS1158 datasheet.

-I will put 9K and 1K resistors in the analog input stage

-Crystal is a product that doesn't need a cap, so I didn't put it in. I will put a cap

-REF5040 is applied as a reference circuit.

-I will be able to put resistor in the SPI line.

In addition, the input stage used was the AD8226 (Rail-to-Rail Output Instrumentation Amp).

I don't think the signal will send the value of -V to the ADC, but I didn't know what to do, so I made a circuit diagram to use the power supply in both ways.

Thank you

AD8226.pdfads1158_16ch.pdf

Hi Bryan

Can you confirm the additional inquiries?

Hi Danny,

I'm sorry, I did not realize you had additional questions based off of your previous post. The explanations you have provided make sense.

Can you please elaborate on what you need help with?

-Bryan

Hi Bryan

I would like you to review if the circuit diagram I made is working.
-AD8226 in front of ADS1158
-The power of ADC is + -2.5V

If there is a problem with this schematic, please let me know
I will reflect

Thank you

Hi Danny,

I am not sure if you intended to include a schematic in a previous post, but the only schematic I have does not include the AD8226. What is the amp used for? Are you using 1x AD8226 per channel? If so, what is the need for the INA, the resistor divider, and the SE-to-diff conversion stage?

There should not be any issue using a +/- 2.5V supply, as long as you stay within the ADC's operating conditions using a bipolar supply. For example, the REF5040 would not be usable if you are going to use a bipolar supply.

-Bryan

Hi Bryan

This circuit diagram is a product that measures 12-channel EMG.
Two +/- electrodes are used per channel
This product receives 2 different muscle exercise values (+/- 2 electrode values) through AD8226.
Then, the voltage difference between the two electrodes is transferred to ADS1158.
The voltage difference between the two electrodes is thought to be a positive value,but it could be a negative value, so the circuit was constructed with a +/- 2.5V power supply.

Shouldn't it be made like this?
Or is it necessary to configure 2 ADS1158 to directly receive 2 electrodes of each channel?

If you think there is a problem with the current circuit configuration, I hope you can share a reference document.

Thank you

Hi Danny,

Thanks for providing further clarification on what you are doing.

Many biopotential measurements (EMG, EEG, ECG) require simultaneous sampling of the inputs - can your system accept multiplexed inputs?

Also, what is the purpose of the resistor divider after the INA? Typically EMG signals are very low voltage e.g. 10mV, so I am curious why you need to divide down the input after gaining it up with the INA? There is a point at which further gain will not produce any tangible benefit, and the added noise from the resistors might actually make your system worse.

Devices like our ADS1194, ADS1196, and ADS1198 are designed specifically for biopotential measurements. They are simultaneous sampling ADCs with low-noise PGAs to gain up low-level signals. You could use an ADS1194 and ADS1198 for 12x electrodes. This might be something else to consider (but if you have questions about those ADCs, please start a new thread).

If you do not need simultaneous sampling and you can explain the need for the resistor divider, then I do believe your system will work. I would also suggest looking at the INA826 if you want a suitable alternative for the AD8226.

-Bryan

Hi Bryan

The system will use multiple inputs

After INA, the resistor divider has made a place in the ADC reference circuit.
So it is only connected to 0R.
If there is no problem with the current circuit diagram, I will remove it later.

Thank you for introducing the ADS119x product
I reviewed it at first, but I couldn't implement 12ch, so I chose ADS1158.
Due to the size issue, it is not currently considered to use two ADS119x.

The INA826 seems to be similar to the AD8226, so let me review

However, I have a question about simultaneous sampling.
ADS1158 is to receive data and process it sequentially in each channel, and is ADS119x to receive and process all channels at once?
It is best to receive all channels at the same time and process them simultaneously.
However, it is possible to think that it is similar to simultaneous sampling if the time is short, although the channels are sequentially input and processed

Thank you

Hi Danny,

It is really up to you to determine if your system needs simultaneously sampled (ADS1194, ADS1198) or multiplexed (ADS1158) inputs.

As I said in my previous post, I would generally expect EMG signals to be simultaneously sampled, since you need to understand how the muscles are behaving at a given moment in time. If there is significant phase delay between measurements, it will not be clear what the muscle groups are actually doing.

The ADS1158 can sample at 23kSPS for 1x channel in auto-scan mode. For 12 channels, that is 23 kSPS / 12x channels = 1.9 kSPS / channel. That is approximately 0.5ms per channel, which represents the time between measurements. Again, this is up to you to decide if that is okay for your application.

-Bryan

Hi Bryan

I think it should be sampled at the same time, but it was designed as of the present considering the product size and various conditions.
If my current design doesn't have a major problem, I will use the auto scan mode.

Is there anything you can say about the other questions above?

Thank you

Hi Bryan

Thanks for the further answers

Actually, this is because I uploaded only a part of the schematic and your first answer to it

Then go back to the beginning, and design it according to the schematic I uploaded, and there is no problem with it?

The first time you answered me like this

• I don't see any input filtering on the ADS1158's analog inputs. This might be necessary for anti-aliasing
• There are no load capacitors shown on the crystal. These are especially important to make sure the crystal oscillates and can actually provide a clock signal to the ADC. You can see that this is a common challenge other ADS1158 users have experienced, so take care to tune your circuit appropriately. We would recommend an external oscillator, as opposed to a crystal, for best performance.
• Looks like you are applying a 4.096V VREF to VREFP since you are using REF5040? But the VREF supply is only 2.5V? If AVDD = 2.5 V, then the voltage on VREFP cannot be greater than AVDD + 0.1V, though I am not really sure what you are trying to do here.
• We usually recommend some small series resistance on the SPI lines for best performance, usually 30-50 ohms

Can I ignore this? Or ask if you can give us additional answers

Thank you

Hi Bryan

I will add a resistance to the SPI line

Once I follow this circuit diagram, I understand that it works basically

After confirming that it works, I will proceed with the optimization.

Thank you