ADS131A02: Buffer amp bandwidth

Hirokazu Takahashi

Part Number: ADS131A02
Other Parts Discussed in Thread: THS4561

Hello Support team,

I have a question about the buffer amplifier at front of ASD131A02.
I will use ADS131A02 under the following conditions.
Fmod (operating clock frequency) = 4.096MHz
OSD (Oversampling Rate) = 256
ODR (output data rate) = 16kHz
Is it correct to think that there is no problem if the bandwidth of the amplifier used in the AD driver circuit in the previous stage is higher than Fmod (4.096MHz)?
If you have any recommended buffer amps, please let me know.

Best Regards,
Hirokazu Takahashi

over 5 years ago

0 Ryan Andrews over 5 years ago

TI__Guru 67886 points

Hello Takahashi-san,

Thank you for your post.

Depending on the application requirements, the amplifier bandwidth should be at least 4x-10x greater than the modulator sampling frequency. This assumes that the amplifier is used in a unity-gain configuration. If the driver stage is configured for gain > 1 V/V, more bandwidth may be required.

The input R-C charge-bucket filter (similar to a SAR ADC circuit) also helps to provide charge to the internal sampling capacitors. The amplifier circuit should be tested with this R-C load to ensure amplifier stability.

The ADS131A02 uses two internal clock dividers to generate the modulator frequency. If the amplifier circuit is designed for 4.096 MHz (max fmod), the circuit will still work well for slower fmod frequencies.

In the past, we have used fully-differential amplifiers (FDAs) like the THS4561 for unbuffered delta-sigma ADC inputs. The input impedance is not very high, so a separate high-impedance buffer might be necessary at the FDA inputs.

Regards,

0 Hirokazu Takahashi over 5 years ago in reply to Ryan Andrews

TI__Expert 8420 points

Hello Ryan-san

Thanks for your prompt reply.
The base bandwidth of input signal is DC for our application and the buffer amp gain will be x1.
We think noise performance is better with higher fmod.
Is it necessary to set the bandwidth of buffer amp at least 4x-10x greater than fmod in that case?
If so, may I ask about the detail reason?

Best Regards,
Hirokazu Takahashi

0 Ryan Andrews over 5 years ago in reply to Hirokazu Takahashi

TI__Guru 67886 points

Hello Takahashi-san,

The amplifier bandwidth requirement is based on the transient voltage settling that happens in the beginning of each modulator period. You can think of this as the acquisition time in a SAR ADC sample-and-hold circuit.

What is the source of the DC voltage that you are measuring? In some applications, we see customers measuring these types of signals without much signal conditioning, so it really depends on the signal characteristics and target accuracy.

Regards,