AD620
5V
20k
Ω
Ω
7
3
8
3k
Ω
3kΩ
REF
IN
G = 100
499
6
AD620B
DIGITAL
DATA
OUTPUT
Ω
3k
Ω
3k
Ω
5
10k
ADC
1
2
4
AGND
AD705
20k
Ω
0.6mA
MAX
1.7mA
0.10mA
1.3mA
MAX
Figure 40. A Pressure Monitor Circuit that Operates on a 5 V Single Supply
Pressure Measurement
Medical ECG
Although useful in many bridge applications, such as weigh
scales, the AD620 is especially suitable for higher resistance
pressure sensors powered at lower voltages where small size and
low power become more significant.
The low current noise of the AD620 allows its use in ECG
monitors (Figure 41) where high source resistances of 1 MΩ or
higher are not uncommon. The AD620’s low power, low supply
voltage requirements, and space-saving 8-lead mini-DIP and
SOIC package offerings make it an excellent choice for battery-
powered data recorders.
Figure 40 shows a 3 kΩ pressure transducer bridge powered
from 5 V. In such a circuit, the bridge consumes only 1.7 mA.
Adding the AD620 and a buffered voltage divider allows the
signal to be conditioned for only 3.8 mA of total supply current.
Furthermore, the low bias currents and low current noise,
coupled with the low voltage noise of the AD620, improve the
dynamic range for better performance.
Small size and low cost make the AD620 especially attractive for
voltage output pressure transducers. Since it delivers low noise
and drift, it will also serve applications such as diagnostic
noninvasive blood pressure measurement.
The value of capacitor C1 is chosen to maintain stability of
the right leg drive loop. Proper safeguards, such as isolation,
must be added to this circuit to protect the patient from
possible harm.
+3V
PATIENT/CIRCUIT
PROTECTION/ISOLATION
R1
10k
R3
0.03Hz
24.9k
Ω
C1
Ω
R
8.25k
HIGH-
PASS
OUTPUT
1V/mV
G
AD620A
G = 143
Ω
R2
24.9k
FILTER
R4
1M
G = 7
Ω
Ω
OUTPUT
AMPLIFIER
AD705J
–3V
Figure 41. A Medical ECG Monitor Circuit
Rev. G | Page 15 of 20
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