noise and coupling components should be connected as
close as possible.
6) Setup of bandwidth and velocity scaling for the optimized crit-
ically damped case should proceed as follows:
Note: DDC has software available to perform the previous calcu-
lations. Contact DDC to request software or visit our web-
site at www.ddc-web.com to download software.
7) Selecting a f
BW
that is too low relative to the maximum appli-
cation tracking rate can create a spin-around condition in
which the converter never settles. The relationship to insure
against spin-around is as follows (TABLE 3):
8) For RDC-19222: package only.
This version is capable of +5V only operation. It accomplishes
this with a charge pump technique that inverts the +5V supply
for use as -5V, hence the +5V supply current doubles. The
built-in -5 V inverter can be used by connecting pin 2 to 26, pin
17 to 22, a 10 µF/10 Vdc capacitor from pin 23 (negative ter-
minal) to pin 25 (positive terminal), and a 47 µF/10 Vdc capac-
- Select the desired f BW (closed loop) based on overall
system dynamics.
- Select f
carrier
≥ 3.5f
BW
- Select the applications tracking rate (in accordance with TABLE 3),
and use appropriate values for R SET and R CLK
- Compute Rv =
- Compute C
BW (pF) =
- Where Fs = 67 kHz for R CLK = 30 KΩ
100 kHz for R CLK = 20 KΩ
125 kHz for R CLK = 15 KΩ
- Compute R
B =
- Compute
3.2 x Fs (Hz) x 10
8
Rv x (f BW)
2
Full Scale Velocity Voltage
Tracking Rate (rps) x 2
resolution
x 50 pF x 1.25 V
0.9
C
BW x f BW
CBW
10
As an example:
Calculate component values for a 16-bit converter with 100Hz
bandwidth, a tracking rate of 10RPS and a full scale velocity
of 4 volts.
- Rv = = 97655 Ω
- Compute C
BW (pF) = = 21955 pF
- Compute R
B = = 410 kΩ
4 V
10 rps x 2
16
x 50 pF x 1.25 V
0.9
21955 x 10
-12
x 100 Hz
3.2 x 67 kHz x 10
8
97655 x 100 Hz
2
6
Data Device Corporation
www.ddc-web.com
RDC-19220 SERIES
Q-05/05-0
itor from -5 V to GND. The current drain from the +5 V supply
doubles. No external -5 V supply is needed (SEE FIGURE 5).
When using the -5 V inverter, the max. tracking rate should be
scaled for a velocity output of 3.5 V max. Use the following equa-
tion to determine tracking rate used in the formula on page 5:
TR (required) x (4.0) = Tracking rate used in calculation
(3.5)
Note: When using the highest BW and Tracking Rates, using
the -5 V inverter is not recommended.
HIGHER TRACKING RATES AND CARRIER FREQUENCIES
Tracking rate (nominally 4 V) is limited by two factors: velocity
voltage saturation and maximum internal clock rate (nominally
1,333,333 Hz). An understanding of their interaction is essential
to extending performance.
The General Setup Considerations section makes note of the
selection of R
v for the desired velocity scaling. Rv is the input resis-
tor to an inverting integrator with a 50 pF nominal feedback capac-
itor. When it integrates to -1.25 V, the converter counts up 1 LSB
and when it integrates to +1.25 V, the converter counts down 1
LSB. When a count is taken, a charge is dumped on the capaci-
tor; such that, the voltage on it changes 1.25 V in a direction to
bring it to 0 V. The output counts per second per volt input is there-
fore:
1
(R
v
x 50 pF x 1.25)
As an example:
TABLE 3. TRACKING/BW RELATIONSHIP
RPS (MAX)/BW RESOLUTION
1 10
0.45 12
0.25 14
0.125 16
RDC-19222
10µf
23
25
+CAP
-CAP
.01uf
(-5c)
-5V
22
17
.01uf
(+5c)
+5V
26
2
47uf 47uf
+
+
FIGURE 5. -5V BUILT-IN INVERTER
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