
ADMC300
–15–
REV. B
register is a 7-bit register so that the ADC sample period is
effectively subdivided into 128 equal time slices. The value
written to the ADCSYNC register is the number of such time
slices before the PWMSYNC pulse that the CONVST pulse is
active. In other words, the occurrence of the CONVST pulse
lags the PWMSYNC pulse of Figure 9 (b) by a time, T
OFFSET
,
that can be expressed as a fraction of the ADC update period:
T
OFFSET
=
(128
ADCSYNC
)
128
ADCDIVn
f
CLKIN
Therefore, for the case where ADCDIVA is 0x180 and ADCSYNC
is 0x060, the CONVST pulse will lag the PWMSYNC pulse by a
quarter of the ADC update period, or 7.68
μ
s, with a 12.5 MHz
CLKIN. The ability to phase shift the ADC update relative to
the PWMSYNC pulse is available only in single update mode of
the PWM.
It is also possible to operate the ADCs at a faster update rate
than the PWM switching frequency and still maintain synchro-
nism, as illustrated in Figure 9 (c). In this example, the value
written to the ADCDIV registers is three times larger than the
value written to the PWMTM register, so that the ADC update
rate is three times faster than the PWM switching frequency.
Synchronism is maintained by setting Bits 7 and 8 of the
ADCCTRL register. In addition, it is possible to introduce a
phase shift between the ADC update and PWMSYNC pulses
while operating at different frequencies, as illustrated by Figure 9
(d). The offset is defined by the ADCSYNC register as a fraction
of the ADC update period in an identical manner to before.
ADC Transfer Characteristics
Each ADC converter of the ADMC300 consists of an input
modulator stage and a decimation filter stage that produces the
final conversion result. The output of the decimation filters are
16-bit, left-aligned, two
’
s complement representation of the
input signal, V
IN
. The ideal ADC transfer characteristics for
both single-ended and differential modes are shown in Figure
10. The transfer characteristics of the ADC when operated in
the differential configuration are shown in Figure 10 (a) and for
the single-ended configuration in Figure 10 (b). The peak-peak
input voltage is 4 V.
The output code of the ADCs is typically given by:
ADCx
=
10,600
×
2.5
V
REFIN
Vx
VxN
(
)
ANALOG
INPUT
3.5 V
V
REF
= 2.5 V
1.5V
0xAD30
0x0000
0x52D0
V1N
V1
ADC CODE
ANALOG
INPUT
4.5 V
V
REF
= 2.5 V
0.5V
0xAD30
0x0000
0x52D0
V1N
V1
ADC CODE
(a)
(b)
Figure 10. (a) Typical Transfer Characteristic of the ADC
in Differential Input Configuration (b) Typical Transfer
Characteristic of the ADC in Single-Ended
PWMTM
PWMCHA
PWMSYNC
AH
CONVST
PWMSYNC
AH
CONVST
PWMSYNC
AH
CONVST
CONVST
AH
PWMSYNC
PWMTM/3
(a)
(b)
(c)
(d)
T
OFFSET
PWMTM/3
PWMTM
PWMCHA
PWMTM
PWMCHA
PWMCHA
PWMTM
T
OFFSET
Figure 9. (a) Synchronization of ADC and PWM at Same
Frequency with No Offset, (b) Synchronization of ADC and
PWM at Same Frequency with Offset, (c) Synchronization
of ADC and PWM with No Offset and ADC Update at
Three Times the PWM Frequency (d) Synchronization of
ADC and PWM at Different Frequencies with Offset (PWM
Is Operating in Single Update Mode)