ADS7040 datasheet(28/43 Pages) TI2

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AVDD

AINP

AINM

GND

AVDD

Device

ROUT

CFLT

Sensor

ADS7040

SBAS676C – NOVEMBER 2014 – REVISED DECEMBER 2015

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9.2.3 8-Bit, 10-kSPS DAQ Circuit Optimized for DC Sensor Measurements

Figure 45. Interfacing the Device Directly with Sensors

In applications where the input is very slow moving and the overall system ENOB is not a critical parameter, a

DAQ circuit can be designed without the input driver for the ADC . This type of a use case is of particular interest

for applications in which the primary goal is to achieve the absolute lowest power possible. Typical applications

that fall into this category are low-power sensor applications (such as temperature, pressure, humidity, gas, and

chemical).

9.2.3.1 Design Requirements

For this design example, use the parameters listed in Table 4 as the input parameters.

Table 4. Design Parameters

DESIGN PARAMETER

GOAL VALUE

Throughput

10 kSPS

SNR at 100 Hz

48.5 dB

THD at 100 Hz

65dB

SINAD at 100 Hz

48 dB

ENOB

7.5

Power

10 µW

9.2.3.2 Detailed Design Procedure

The ADS7040 can be directly interfaced with sensors at lower throughputs without the need of an amplifier

buffer. The analog input source drive must be capable of driving the switched capacitor load of a SAR ADC and

settling the analog input signal within the acquisition time of the SAR ADC. However, the output impedance of

the sensor must be taken into account when interfacing a SAR ADC directly with sensors. Drive the analog input

of the SAR ADC with a low impedance source. The input signal requires more acquisition time to settle to the

desired accuracy because of the higher output impedance of the sensor. The simplified circuit for a sensor as a

voltage source with output impedance (ROUT) is shown in Figure 45.

The acquisition time of a SAR ADC (such as the ADS7040) can be increased by reducing throughput in the

following ways:

1. Reducing the SCLK frequency to reduce the throughput, or

2. Keeping the SCLK fixed at the highest permissible value (that is, 12 MHz for the device) and increasing the

CS high time.

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