TDA8575 Datasheet, PDF (5/16 Pages) NXP Semiconductors – Ground noise isolation amplifier

Philips Semiconductors
Ground noise isolation amplifier
Preliminary specification
TDA8575
FUNCTIONAL DESCRIPTION
System description
To enable a high common mode rejection a new system
setup is used. The voltage to current converter, referred to
as V → I in the block diagram of Fig.1, replaces the
resistors that can be seen in the conventional system
solution.
Both systems are shown in Figs 4 and 5. In the
conventional system the common mode rejection is limited
by the matching properties of the resistors resulting in a
CMRR of 60 dB maximum. Using the new system setup a
CMRR of 80 dB is achieved.
This is inconvenient for most applications and therefore
the TDA8575(T) is equipped with a quick charge circuit.
On power-on the quick charge circuit charges the
capacitor C2 connected to the IN- pins. The quick charge
circuit consists of a voltage buffer and a control circuit
(referred to as ‘reference and power check’ in Fig.6) that
monitors the supply voltage V_CC. If the supply voltage rises
more than ≈ 2 V the voltage buffer is switched on.
After charging C2 the voltage buffer is switched off.
The charge time of C2 will equal the charge time of C4, the
SVRR capacitor.
Power on
In Fig.6 the preferred input capacitor values are shown.
If the capacitor C2 = 22 µF connected to the IN- inputs had
to be charged by the 0.5V_cc voltage source a charge time
of 5τ = 5 × -3---6---0--2---k----^Ω--- × 22 µF = 20s would be required.
handbook, halfpage
Vi
Vo
0.5 VCC
MGE830
handbook, halfpage
Vi
VI
Vo
IV
0.5 VCC
MGE831
Fig.4 Conventional system.
Fig.5 New system using V → I converters.
1996 Jul 29
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