AD8420ARMZ AD [Analog Devices], AD8420ARMZ Datasheet - Page 8

• Manufacturer Part Number: AD8420ARMZ
• Description: Wide Supply Range, Rail-to-Rail Output Instrumentation Amplifier
• Manufacturer: AD [Analog Devices]

AD8420
THEORY OF OPERATION
ARCHITECTURE
The AD8420 consists of three amplifiers: two matched
transconductance amplifiers that convert voltage to current and
one integrator amplifier that converts current to voltage.
The AD8420 works as follows: assume a differential voltage is
applied across inputs +IN and -IN. This input voltage is
converted into a current by Amplifier A1. This will create a
difference in current between A1 and A2, which is fed into A3.
A3’s output voltage will change until A2 sinks all the current A1
is generating. Because the gain of A1 and A2 are matched, this
means the differential input voltage across A1 will appear across
the inputs of A2. Gain is set by the ratio of R2 to R1.
Because the AD8420 converts the input differential signals to a
current, there are no internal headroom issues as with
traditional instrumentation amplifier architectures. This is
particularly important when amplifying a signal with a
common mode voltage near one of the supply rails.
To improve robustness and ease of use, the AD8420 includes
overvoltage protection on its inputs. This protection scheme
allows input voltages well beyond the supply rails (as well as
wide differential input voltages) without damaging the part.
SETTING THE GAIN
The transfer function of the AD8420 is
where:
G
=
1
V
+
OUT
R2
R
1
= G(V
IN+
− V
IN−
+IN
–IN
) + V
REF
OVERVOLTAGE
OVERVOLTAGE
PROTECTION
PROTECTION
ESD AND
ESD AND
V
A1
I
Figure 3. Simplified Schematic
Rev. PrD | Page 8 of 8
V
BIAS
Table 7. Suggested Resistors for Various Gains - 1% Resistors
R1 (kΩ)
none
49.9
20
10
5
2
1
1
1
1
While the ratio of R2 to R1 sets the gain, the absolute value of
the resistors is up to the designer. Larger values reduce power
consumption and output loading; smaller values limit FB input
bias current error.
A method that allows large value feedback resistors while
limiting FB bias current error is to place a resistor of value
R1||R2 in series with the REF terminal as shown in Figure 4. At
higher gains, this resistor can simply be the same value as R1.
Figure 4. Cancelling Out Error from FB Input Bias Current
–IN
+IN
A3
AD8420
A2
V
G = 1+
I
R2
R1
+VS
–VS
+VS
–VS
Preliminary Technical Data
V
REF
FB
R2 (kΩ)
short
49.9
80.6
90.9
95.3
97.6
100
200
499
1000
OUT
R1||R2
R2
R1
V
REF
FB
OUT
R2
R1
V
REF
Gain
1.00
2.00
5.03
10.09
20.06
49.8
101
201
500
1001