Hi everyone
I want to convert a bipolar signal from +3v to -3v using an adc in single ended mode of operation.Sampling rate of adc should be around 2ksps.Atleast 12 bit resolution,i2c/spi interface,atleast 4 channels should be available.I am using ADS1115 adc for this purpose.but I am not sure that this adc will convert the negative range of my signal or not.If not then which ADC will be appropriate for my application.Please help me with this.
Thanks and regards
Arun
Hi Arun,
I think that the ADS1115 could work for your application, but you may need some extra circuitry. This would depend on your input signal. If your inputs are outside the range of the ADC input, you'll need to level shift it and then scale it to fit the range of the ADC.
Here's the specification that is most important to your question:
The analog input needs to be GND and VDD. If your input goes below GND to -3V, then yes, the ADC will not be able to convert that part of the signal. In most ADCs, you will need to have the signal within this or a similar range for proper operation. Some ADCs will have a slightly more restricted range, so you may need to be careful in choosing the device.
If your single-ended input goes from -3V to +3V, you'll need to scale down the signal, because the input max spans 6V, which is larger that the GND to VDD range anyway. Using an op-amp and some resistors you should be able to scale the input by 1/2 and level shift the signal so that it is within the input range of the ADC.
While the multiplexer section on page 12 of the ADS1115 datasheet explains some of the parameters of single ended operation, page 26 shows a bit more with a diagram. The CONFIG register controls the multiplexer and you can find information on that on pages 18 and 19.
Joseph Wu
Hi Arun,
There are a couple of things to point out:
The ADS1115 has a max data rate of 860sps. This is significantly less than the 4ksps that you're looking for. If you only need 12 bits of accuracy, you can look at the ADS1015 instead.
Converting the bipolar input around GND to something higher isn't difficult, but it does require some additional circuitry. Below, I have a circuit that converts the input of -3V to +3V centered at GND, to an input of -1.5V to +1.5V centered at 2.048V. You will need to use some precision resistors, but it is something that can be done. You can simulate this to see that it fits your needs, but remember that the resistors should match well for good common mode rejection,
If your input only operates from -1V to +1V, then you might want to consider the ADS1120. The analog supplies can be run from -2.5V to +2.5V (which you would need to add to your circuit), while the digital supplies can be run from 0V to 5V or lower. This device also runs at a data rate of 2ksps is faster than the ADS1115. Similarly, the ADS1148 can be run from a split analog and digital supplies. It has some extra inputs and other extra features you might not need, but it also runs up to 2ksps.
In both the above devices, you could use the same reference that used for the the REF on the device to bias one of the input lines.
If you need help choosing a device, start by listing all the specifications that you need, and outline which of those are not as critical. You can post it back to the forum.
Joseph Wu
