AD8233ACBZ-R7 datasheet(26/30 Pages) AD

26 / 30 page

Data Sheet

AD8233

Rev. 0 | Page 25 of 29

Driving ADCs

The ability of AD8233 to drive capacitive loads makes it ideal

for driving an ADC without an additional buffer. However,

depending on the input architecture of the ADC, a simple, low-

pass RC network may be required to decouple the transients

from the switched capacitor input typical of modern ADCs.

This RC network also acts as an additional filter that can help

reduce noise and aliasing. Follow the recommended guidelines

from the ADC data sheet for the selection of proper R and C

values. Table 7 lists compatible ADCs by category.

Table 7. Compatible ADCs by Category

Analog-to-

Digital

Converters

Microcontrollers

Optical

Sensors

Accelerometers

AD7091

ADuCM350

ADPD103

ADXL363

AD7988-1

ADPD105

ADC

AD8233

Figure 69. Driving an ADC

DRIVEN ELECTRODE

A driven lead (or reference electrode) is often used to minimize

the effects of common-mode voltages induced by the power line

and other interfering sources. The AD8233 extracts the common-

mode voltage from the instrumentation amplifier inputs and

makes it available through the RLD amplifier to drive an opposing

signal into the patient. This functionality maintains the voltage

between the patient and the AD8233 at a near constant, greatly

improving the CMRR.

As a safety measure, place a resistor between the RLD pin

(Pin D5) and the electrode connected to the subject to ensure

that current flow never exceeds 10 μA. Calculate the value of

this resistor to be equal to the supply voltage across the AD8233

divided by 10 μA.

The AD8233 implements an integrator formed by an internal

150 kΩ resistor and an external capacitor to drive this electrode.

The choice of the integrator capacitor is a trade-off between line

rejection capability and stability. It is recommended that the

capacitor be small to maintain as much loop gain as possible,

around 50 Hz and 60 Hz, which is typical for line frequencies.

For stability, it is recommended that the gain of the integrator

be less than unity gain at the frequency of any other poles in the

loop, such as those formed by the capacitance and the safety

resistors of the patient. The suggested application circuits use a

1 nF capacitor, which results in a loop gain of about 20 at line

frequencies, with a crossover frequency of about 1 kHz.

In a 2-lead configuration, the RLD pin (Pin D5) amplifier can

be shut down or used to drive the bias current resistors on the

inputs. Although not as effective as a true driven electrode, this

configuration can provide some common-mode rejection

improvement if the sense electrode impedance is small and well

matched.

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