TCM38C17IDL Datasheet, PDF (18/22 Pages) Texas Instruments – QComboE FOUR-CHANNEL QUAD PCM COMBO

TCM38C17IDL
QCombo™ FOUR-CHANNEL (QUAD) PCM COMBO
SLWS040C – JUNE 1996 – REVISED OCTOBER 1999
APPLICATION INFORMATION
Various TCM38C17IDL output configurations are detailed in the following paragraphs.
differential configuration
For connection to a transformer, the fully differential configuration is recommended to provide maximum
possible output, or voltage swing, to the primary of an attached transformer. Figure 9 shows the QCombo in a
fully-differential mode.
PWRO +
GSR
PWRO –
VO ₊
R1
RL
Vd
R2
VO _–
Figure 9. Fully Differential Gain-Setting Configuration
PWRO+ and PWRO – are low-impedance complementary outputs. The total output available for the output load
(R_L) is then V_D = V_O+ – V_O– . R1 and R2 form a gain-setting resistor network with a center tap connected to the
GSR input.
R1 + R2 should be greater than 10 kΩ and less than 100 kΩ because the parallel combination R1 + R2 and R_L
sets the total loading. The total parasitic capacitance of the GSR input, along with the parallel combination of
R1 and R2, define a time constant that must be minimized to avoid inaccuracies in the gain calculations.
The resistor gain control actually consists of attenuating the full differential output voltage. The equation to
determine the value of the attenuation constant is given in equation 1.
+ ^{)) BB} A
1
4
(R1
(R1
R2)
R2)
(1)
which can also be expressed as shown in equation 2.
+ _{)) B} A
R1 R2
4(R2 R1 4)
(2)
where A = attenuation constant
Depending on the values of gain setting resistors R1 and R2, the attenuation constant (A) can have a value of
0.25 to unity (1), or approximately 12 dB of voltage adjustment.
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