REF5045 datasheet(41/62 Pages) TI2

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ADS8881

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SBAS547D – MAY 2013 – REVISED AUGUST 2015

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11.2.3.1 Design Requirements

Design an application circuit optimized for using the ADS8881 to achieve

•

> 98.5-dB SNR, < –110-dB THD and

•

± 1.5-LSB linearity and

•

maximum specified throughput of 1 MSPS

11.2.3.2 Detailed Design Procedure

The application circuits are shown in Figure 69 and Figure 70. In both applications, the input signal is processed

through a high-bandwidth, low-distortion, fully-differential amplifier (FDA) designed in an inverting gain

configuration and a low-pass RC filter before being fed into the ADC.

The reference driver circuit, shown in Figure 69 and Figure 70, generates a voltage of 4.5 V dc using a single 5-

V supply. This circuit is suitable to drive the reference of the ADS8881 at higher sampling rates up to 1 MSPS.

The reference voltage of 4.5 V in this design is generated by the high-precision, low-noise REF5045 circuit. The

output broadband noise of the reference is heavily filtered by a low-pass filter with a 3-dB cutoff frequency of 160

Hz.

The reference buffer is designed with the THS4281 and OPA333 in a composite architecture to achieve superior

dc and ac performance at a reduced power consumption, compared to using a single high-performance amplifier.

The THS4281 is a high-bandwidth amplifier with a very low output impedance of 1 Ω at a frequency of 1 MHz.

The low output impedance makes the THS4281 a good choice for driving a high capacitive load to regulate the

voltage at the reference input of the ADC. The high offset and drift specifications of the THS4281 are corrected

by using a dc-correcting amplifier (OPA333) inside the feedback loop. The composite scheme inherits the

extremely low offset and temperature drift specifications of the OPA333.

As a rule of thumb, the distortion from the input driver must be at least 10 dB less than the ADC distortion. The

distortion resulting from variation in the common-mode signal is eliminated by using the FDA in an inverting gain

configuration that establishes a fixed common-mode level for the circuit. This configuration also eliminates the

requirement of a rail-to-rail swing at the amplifier input. Therefore, these circuits use the low-power THS4521 as

an input driver that provides exceptional ac performance because of its extremely low-distortion and high-

bandwidth specifications.

In addition, the components of the antialiasing filter are such that the noise from the front-end circuit is kept low

without adding distortion to the input signal.

The circuit in Figure 69 shows a fully-differential DAQ block optimized for low distortion and noise using the

THS4521 and ADS8881. This front-end circuit configuration requires a differential signal at the input of the FDA

and provides a differential output to drive the ADC inputs. The common-mode voltage of the input signal provided

to the ADC is set by the VOCM pin of the THS4521 (not shown in Figure 69). To use the complete dynamic range

of the ADC, VOCM can be set to VREF / 2 by using a simple resistive divider. However, note that the ADS8881

allows the common-mode input voltage (VCM) to be set to any value in the range of 0 V to VREF.

The circuit in Figure 70 shows a single-ended to differential DAQ block optimized for low distortion and noise

using the THS4521 and the ADS8881. This front-end circuit configuration requires a single-ended ac signal at

the input of the FDA and provides a fully-differential output to drive the ADC inputs. The common-mode voltage

of the input signal provided to the ADC is set by the VOCM pin of the THS4521 (not shown in Figure 70). To use

the complete dynamic range of the ADC, VOCM can be set to VREF / 2 by using a simple resistive divider.

However, note that the ADS8881 allows the common-mode input voltage (VCM) to be set to any value in the

range of 0 V to VREF.

REF5045 Datasheet(PDF) 41 Page - Texas Instruments

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