z89138 ZiLOG Semiconductor, z89138 Datasheet - Page 54

• Manufacturer Part Number: z89138
• Description: Voice Processing Controllers
• Manufacturer: ZiLOG Semiconductor

Z89138/Z89139
Voice Processing Controllers
Figure 42 shows the input circuit of the ADC. When con-
version starts, the analog input voltage from the input is
connected to the MSB and LSB flash converter inputs as
shown in the Input Impedance CKT diagram. Shunting 31
parallel internal resistances of the analog switches and si-
multaneously charging 31 parallel 1 pF capacitors is equiv-
alent to a 400 Ohms input impedance in parallel with a 31
54
Input Sample
Notes:
1. SCLK = 10 MHz (XTAL =
20.48 MHz)
A/D Result
DSP Write
DSP INT
SCLK
P32
1
1
2
3
C Parasitic
R Source
4
5
Figure 42. Input Impedance of ADC
6
Figure 41. ADC Timing Diagram
7
on Resistance
CMOS Switch
P R E L I M I N A R Y
8
2 - 5 k
V Ref
V Ref
V Ref
9
10
pF capacitor. Other input stray capacitance adds about 10
pF to the input load. Input source resistances up to 2 kO-
hms can be used under normal operating conditions with-
out any degradation of the input settling time. For larger in-
put source resistance, longer conversion cycle times can
be required to compensate the input settling time problem.
V
11
C .5 pF
C .5 pF
C .5 pF
REF
is set using the V
26
27
28
29
31 CMOS Digital
REF
Comparators
30
+ pin.
31
32
DS97TAD0201
Zilog