AD830
TRAD ITIO NAL D IFFERENTIAL AMP LIFICATIO N
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. T he basic function VO = V1–V2 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.
AD 830 FO R D IFFERENTIAL AMP LIFICATIO N
T he 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. T he circuit of Figure 20 shows how
simply the AD830 is configured to produce the difference of two
signals V1 and V2, 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.
R
R
1
2
V
2
V
1
2
V→ I
V
R
I
3
V
X
Y
OUT
V
1
A=1
V
OUT
ONLY IF R = R = R = R
R
1
2
3
4
4
DOES
V
= V – V
OUT 1 2
I
V→ I
Figure 19. Op Am p Based Difference Am plifier
V
= V – V
1 2
OUT
P RO BLEMS WITH TH E O P AMP BASED AP P RO ACH
• Low Common-Mode Rejection Ratio (CMRR)
• Low Impedance Inputs
Figure 20. AD830 as a Difference Am plifier
• 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 R1–R4 to Achieve
High CMRR
• Halves the Bandwidth of the Op Amp
• High Power Dissipation in the Resistors for Large Common-
Mode Voltage
AD VANTAGEO US P RO P ERTIES O F TH E AD 830
• 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
ADI [ ADI ]
