REV. A

–12–

AD8225

Driving a High Resolution ADC

Most high precision ADCs feature differential analog inputs.

Differential inputs offer an inherent 6 dB improvement in S/N

ratio and resultant bit resolution. These advantages are easy to

realize using a pair of AD8225s.

AD8225s can be configured to drive an ADC with differential

inputs by using either single-ended or differential inputs to the

AD8225s. Figure 7 shows the circuit connections for a differen-

tial input. A single-ended input may be configured by connecting

the negative input terminal to ground.

2

3

2

3

6

6

5

5

AD8225

AD8225

OP177

1.25V

4.99k

4.99k

2.7nF

2.7nF

75

75

AD7675

100kSPS

5V

AD780

2.5V

RERERENCE

+IN

–IN

ALTERNATE

CONNECTION

FOR SE SOURCE

Figure 7. Driver for Differential ADC

The AD7675 ADC illustrated in Figure 7 is a SAR type converter.

When the input is sampled, the internal sample-and-hold capacitor

is charged to the input voltage level. Since the output of the

AD8225 cannot track the instantaneous current surge, a voltage

glitch develops. To source the momentary current surge, a

capacitor is connected from the A/D input terminal to ground.

Since the AD8225 cannot tolerate greater than approximately

100 pF of capacitance at its output, a 75

Ω series resistor is

required at each in amp output to prevent oscillation.

Using the Reference Input

Note in the example in Figure 7 that Pin 5, the reference input, is

driven by a voltage source. This is because the reference pin is

internally connected to a 15 k

Ω resistor, which is carefully trimmed

to optimize common-mode rejection. Any additional resistance

connected to this node will unbalance the bridge network formed

by the two 3 k

Ω and two 15 kΩ resistors, resulting in an error

voltage generated by common-mode voltages at the input pins.

AD8225 Used as an EKG Front End

The topology of the instrumentation amplifier has made it the

circuit configuration of choice for designers of EKG and other

low level biomedical amplifiers. CMRR and common-mode

voltage advantages of the instrumentation amplifier are tailor

made to meet the challenges of detecting minuscule cardiac

generated voltage levels in the presence of overwhelming levels

of noise and dc offset voltage. The subtracter circuit of the in

amp will extract and amplify low level signals that are virtually

obscured by the presence of high common-mode dc and ac

potentials.

A typical circuit block diagram of an EKG amplifier is shown in

Figure 8. Using discrete op amps in the in amp and gain stages,

the signal chain usually includes several filters, high voltage

protection, lead-select circuitry, patient lead buffering, and an

ADC. Designers who roll their own instrumentation amplifiers

must provide precision custom trimmed resistor networks and

well matched op amps.

The AD8225 instrumentation amplifier not only replaces all the

components shown in the highlighted block in Figure 8, but also

provides a solution to many of the difficult design problems

encountered in EKG front ends. Among these are patient gener-

ated errors from ac noise sources and errors generated by unequal

electrode potentials. Alone, these error voltages can exceed the

desired QRS complex by orders of magnitude.

INSTRUMENTATION AMPLIFIER

G = 3 TO 10

LEAD

SELECT,

HV

PROTECTION,

FILTERING

GAIN AND ADC

TOTAL G = 1000

PATIENT

ISOLATION

BARRIER

DIGITAL DATA

TO SYSTEM

MAINFRAME

A1

A2

A3

Figure 8. Block Diagram, EKG Monitor Front End Using Discrete Components