ADS8684: Query regarding differential signal measuring using ADC

Hi Jishnu,

ADS8684 only supports single-ended(SE) signals, either bipolar (+/-10.24V or +/-5.12V, +/-2.56V) or unipolar (0-10.24V or 0-5.12V). You still external instrumentation amplifiers or other circuits for differential to single-ended signal conversion. 

The internal PGA is implementing the signal conditioning from SE HV bipolar to proper signal range for internal ADC core.

I hope this helps.

Best regards,

Dale

Hi Dale,

Thank you for your response & Sorry for misleading by the term "differential". In my case what I want to measure is a Bipolar signal which ranges between -10V and +10V.

So I think I need not convert the signal to SE. Should I use and external Instrumentation amplifier to Improve the signals or should I feed the input directly to the ADC inputs. The usage of an external instrumentation amplifier without -ve supply will remove the -ve range of the signal, won't it? so how should I use this? 

Thank you

Hi Jishnu,

Can you confirm your +/-10V signal is a single-ended signal and not a differential +/-10V signal? If it's a single-ended signal, you do not need external instrumentation amplifier or other amplifier. Instrumentation amplifier can be easily used to convert a differential signal with a common-mode voltage to a single-ended output with low offset, low noise, easy and high gain setting.

Best regards,

Dale

Hi Dale,

I will verify my signal is really single ended or not.

In case of differential, If I am using an Instrumentation amplifier without negative power supply, I wont get the feature to measure negative ranges of signal right? So can you suggest a way to implement this without need of negative supply?

Hi Jishnu,

Update: let me explain it a little bit. Below are examples based on INA828 instrumentation amplifier:

1. When +/-15V power supplies are applied to the INA828 and the input signal is a differential +/-10V (Vina_in), the output of INA828 is +/-10V when the Gain is 1. The full dynamic range (-10V~10V) of ADS8684 is utilized. Both your positive and negative signals can be measured. See the timing from the TI-TINA simulation below.

2. When only +15V power supply is applied to the INA828, your input signal should be limited to a differential +/-5V (Vina_in) because of +15V supply. The output of INA828 is 0~10V when the Gain is 1 and the bias voltage(Vref) is 5V.  See the timing from the TI-TINA simulation below.  The full dynamic range (-10V~10V) of ADS8684 is only utilized for half (0~10V). Both your positive and negative signals can still be measured, however your actual resolution of ADC is only 15-bit. If you want to support -10V~10V differential signal, the single power supply of INA828 should be higher than 20V. 

I attached these two simulation files, you can modify them and do some experiments in TI-TINA.

To utilize the full dynamic range of ADS8684 to measure your differential -10V~10V signal, dual power supplies for INA828 are required. TI has some power devices which can be used to simply generate a negative power supply, you can check or ask on E2E forum for power devices.

If your signal is single-ended +/-10V, you can simply connect it to the ADS8684 for measurement.

Best regards,

Dale

/cfs-file/__key/communityserver-discussions-components-files/73/INA828_5F00_Dual-Power-Supply_5F00_TINA_2D00_TI.TSC

/cfs-file/__key/communityserver-discussions-components-files/73/INA828_5F00_Single-Power-Supply_5F00_TINA_2D00_TI.TSC

Hi Dale,

Thank you for the clarification. Today I confirmed my signal is Single ended Bipolar, so I will use the design without INA.

I have one more doubt.

In my case the frequency may become greater than 15 KHz, In datasheet I found there is an internal LPF which has a cutoff of 15KHz. Can I Adjust the frequency of the Internal LPF. Or how can I use the device to measure such signals

Thanks & Regards

Jishnu

Hi Jishnu,

No, you can not change the BW of internal LPF. Also, when you use this ADC to measure a high frequency signal (e.g. 15kHz), your signal will be attenuated and distorted by the LPF. There are two possible solutions:

1. Use an ADC with integrated AFE and high impedance like ADS8684 but has a higher BW of LPF. ADS8686S has a 376kHz BW of internal LPF, however it has 16 input channels.

2. Use a conventional ADC without LPF, however an high BW amplifier is required to drive the ADC on each channel. Please see some ADCs in the following link. For example, AD7067: 16-bit, 8-ch, 800ksps with multiplexer.

/cfs-file/__key/communityserver-discussions-components-files/73/ADC_5F00_16_2D00_bit_5F00_8_2600_4_2D00_ch-without-AFE-and-LPF.xlsx

Best regards,

Dale