AD830
TRADITIONAL DIFFERENTIAL AMPLIFICATION
AD830 FOR DIFFERENTIAL AMPLIFICATION
In the past, when differential amplification was needed to reject
common-mode signals superimposed with a desired signal; most
often the solution used was the classic op amp based difference
amplifier shown in Figure 19. The basic function V
O
= V
1
–V
2
is
simply achieved, but the overall performance is poor and the cir-
cuit possesses many serious problems that make it difficult to re-
alize a robust design with moderate to high levels of
performance.
R
1
V
2
R
2
The AD830 amplifier was specifically developed to solve the
listed problems with the discrete difference amplifier approach.
Its topology, discussed in detail in a later section, by design acts
as a difference amplifier. The circuit of Figure 20 shows how
simply the AD830 is configured to produce the difference of two
signals V
1
and V
2
, in which the applied differential signal is
exactly reproduced at the output relative to a separate output
common. Any common-mode voltage present at the input is
removed by the AD830.
V
1
V
→
I
I
X
A=1
I
Y
V
→
I
V
OUT
= V
1
– V
2
V
OUT
R
3
V
1
R
4
V
2
V
OUT
ONLY IF R
1
= R
2
= R
3
= R
4
DOES
V
OUT
= V
1
– V
2
Figure 19. Op Amp Based Difference Amplifier
PROBLEMS WITH THE OP AMP BASED APPROACH
• Low Common-Mode Rejection Ratio (CMRR)
• Low Impedance Inputs
• CMRR Highly Sensitive to the Value of Source R
• Different Input Impedance for the + and – Input
• Poor High Frequency CMRR
• Requires Very Highly Matched Resistors R
1
–R
4
to Achieve
High CMRR
• Halves the Bandwidth of the Op Amp
• High Power Dissipation in the Resistors for Large Common-
Mode Voltage
Figure 20. AD830 as a Difference Amplifier
ADVANTAGEOUS PROPERTIES OF THE AD830
• High Common-Mode Rejection Ratio (CMRR)
• High Impedance Inputs
• Symmetrical Dynamic Response for +1 and –1 Gain
• Low Sensitivity to the Value of Source R
• Equal Input Impedance for the + and – Input
• Excellent High Frequency CMRR
• No Halving of the Bandwidth
• Constant Power Distortion vs. Common-Mode Voltage
• Highly Matched Resistors Not Needed
–8–
REV. A
AD [ ANALOG DEVICES ]
