Hello,

ADS8168 is a 16 bit, 8 ch device with an integrated reference and is similar to LTC1867L. It comes in a VQFN (32) 5.0mm x 5.0mm package.

ADS8028 is a 12 bit, 8 ch device with an integrated reference that comes in a Thin QFN-20 4.0mm x 4.0mm package

Based on your requirement of 16 bit resolution, ADS8168 is the closest match. Please let us know if you have any further questions.

Thanks
Ravi Raghavendra

Hi Ravi,

Thanks for your reply. The package of ADS8168 is very large, I cannot use it. And ADS8028 cannot meet my requirement.

I would like to explain my application.

The input signal of LTC1867L is the output of the op amp. VCC is 3.3V for my whole circuit system.

The input signal is maybe 1na~1uA. Then I use the transimpedance amp to convert the current signal to the voltage signal.
After this, I use the non-inverting circuit to amplify the voltage signal. That mean I use the 2 stage amplify circuit.
At last, the output signal is input to the input of LTC1867L.

My questions are below:

Q1: I do not know how to get the output signal frequency of the op amp.

If I get the output signal frequency of the op amp, I can select the ADC that the sample rate is higher 2 times than

the output signal frequency of the op amp. Is this correct?

Q2: Since the output signal of the op amp is DC signal. Is the output signal frequency of the op amp equal 0HZ?

If the output signal frequency of the op amp equal 0HZ, then I can select a very low sample rate and 20bit or 24bit
ADC to replace LTC1867L, correct?

Can I use the oscilloscope to capture the output signal frequency of the op amp?

Q3: You may select the 20bit or 24bit device for the replacement. VCC is 3.3V, The package should be less than 3mm*3mm.

The reference should be 2.048V that can be the internal reference or the external voltage.

For all requirements, would you help me to select a suitable device?

Q4: I have a voltage conversion sample question.

For example, if I use a 12bit ADC and the reference is 2.048V, and the ADC read the data is 3000.

a. What is the input analog voltage value? Can you give me a calculation formula?


b. If the input signal is 2.5V, what is the value of the digital for 2.5V analog signal? Can you give me a calculation formula?

2.5*Vref/(2*12-1=4095)=2.5*2.048/4095?

Hello,

The smallest ADC package that meets your requirements of 16b, internal reference, and 8 channels is the 5x5 VQFN package. The ADS8168 that Ravi suggested is a good example.

Since you are measuring a very low bandwidth signal (~0Hz or DC), you could use a much lower data rate ADC, such as a delta-sigma. The ADS114S08 is in the same 5x5 VQFN package, but it includes a PGA, which may eliminate the 2nd stage gain amplifier in your signal chain. This could reduce the overall solution size.

Another option would be to use two 4 channel ADCS. The ADS1118 is available in a 2x1.5mm package, and includes a PGA and internal reference. If you need higher performance, the ADS1119 is available in a 3x3mm package, and also includes a PGA and internal ref.

Answers to your specific questions.
Q1: The output bandwidth is determined by your input signal bandwidth requirements. Since you are measuring near 0Hz, you do not need high BW amplifiers. In your case, regardless of the amplifier used, you will want to use a low pass filter to limit the bandwidth, and the total noise of the system that the ADC will see at its inputs. I suggest using a low pass filter around 10Hz to reject 50/60Hz power line noise, as well as higher frequencies. If you sample at 500Hz or higher, and then average (or use a delta-sigma), you should get a very clean signal.
Q2: The output signal of the opamp will be your input signal (~0Hz) plus noise, which will have a bandwidth equal to whatever amplifier configuration you use.
Q3: See above alternative ADC suggestions.
Q4: This depends on the specific ADC that you use. There are several different input configurations which will effect the transfer function. However, each ADC should have this specified somewhere in the datasheet. As an example, the ADS7042 is a 12b device with the following transfer function: Vin=CODE*VREF/2^N.

If using a 2.048V reference and the data is 3000, then Vin for the ADS7042 will equal Vin=3000*2.048/2^12=1.500V.
If the input signal is greater than the reference voltage (2.5Vin > 2.048Vref), the code value will ideally read 4095 (or 0xFFF) for full scale. However, once you exceed the full scale range, you need to make sure you are not violating the ABS max input specs.

TI has some excellent online videos that discuss these concepts. Please take a look at the TI Precision Lab series.

training.ti.com/ti-precision-labs-overview

Start with the ADC series, but all of these have some great information.

Thank You,
Keith N.
Precision ADC Applications

Hi Keith,

Thanks for your reply.

I still cannot understand.

Q1: As you said, for ADS7042, if the reference is 2.048V and the signal is greater than the reference (2.5Vin > 2.048Vref),

the code value will ideally read 4095 (or 0xFFF).

This mean ADC cannot convert 2.5V to a valid code digital signal when the reference is 2.048V, correct?

This also mean the input signal voltage cannot be higher than the reference voltage, correct?

But from the datasheet, the input voltage can be from 0 to AVDD. This mean the input voltage can be allowed higher than the
reference. So, I do not quite understand. Would you explain this? The relationship between the reference voltage and the input signal range.

Q2: What is ABS max input specs that you said?

Q3：The input voltage should be 1LSB~(VREF-LSB) for (ADS7042 , correct?  I refer to table 1 of the datasheet.

Q3: If I still convert 2.5V input signal for ADS7042, the only way is to increase the reference voltage to make sure the input
signal voltage is less than the reference voltage. Then ADS7042 can convert 2.5V to a valid code value, correct?
We need to keep the input signal voltage is less than the reference, then ADC can convert the input signal correctly, correct?


Q4: I would like to select ADS122C04. Does it meet my requirement?

Q5: I want to confirm the input signal range for ADS122C04. If VCC is 3.3V and the reference is 2.048V,

the input signal voltage range is 0~(FS (223 – 1) / 223)? Or -FS~(FS (223 – 1) / 223)? I refer to  table 15 of the datasheet.