Down scale in the case of OPA1632 + ADS1281

Yu-Hung

Expert 1130 points

Other Parts Discussed in Thread: OPA1632, ADS1281, TINA-TI

Hi,

I'm try to use ADS1281EVM.

And the circuit is shown as following:

The OPA1632 is the preamplifier

My questions are:

1. How is the circuit function of OPA1632? LPF or buffer?

If it is LPF, how to calculate cut-off frequency? Is it has formula?

2. The analog input range of ADS1281 is +2.5V to -2.5V.

But the max. analog output of seismic sensor is +10V to -10V.

Is it possible to modify the circuit of OPA1632 which is shown in the figure to down scale the input range to suitable for ADS1281? (Gain = 0.25)

In most cases usually down scale by resistance voltage divider.

I think it is not suitable for the precision measurement, right?

3. Why the power supply of OPA1632 given by +10V and -10V?

In the data sheet, the supply range is : ±2.5V to ±16V.

Why the circuit do not use AVDD/AVSS(±5V) but extra power(±10V)?

Thanks.

YHH

0 Christopher Hall over 10 years ago

Hi Yu-Hung,

The OPA1632 is configured in as a buffer, but it also provides a LPF response as it runs out of bandwidth. The GBW product will give you the approximate cut-off frequency.
By changing the feedback resistor values you can create a gain of 0.25 V/V, as shown below:

A voltage divider may work in a precision application, it would depend on the resistor performance specification. There are trade-offs between using a passive vs active attenuator circuit.
The common-mode range of the OPA1632 does not allow for signals near the supply rails. By increasing the supply voltage, there is more flexibility with the input signal range.