MAX1474 Datasheet, PDF (5/8 Pages) Maxim Integrated Products – Miniature Electronically Trimmable Capacitor

Miniature Electronically Trimmable Capacitor
While EN is asserted, the internal counter counts posi-
tive edges of DAT. The number of sequential pulses on
DAT determines the capacitance setting. As EN is de-
asserted, the counter value is latched into the capaci-
tance control registers first, and then the counter is set
to zero. The programmed capacitance value then
appears between CP and CM. During the period when
EN is asserted, the capacitance value between CP and
CM stays unchanged. Figure 1 is a digital timing dia-
gram.
Q Enhancement Circuit
Since the capacitors have the multiplexer series resis-
tance associated with them, the quality factor of the
trimmable capacitors is not high. For example, a 10pF
capacitor only has a theoretical Q of about 10 at
315MHz with 5Ω series resistance. As in Figure 2, an
impedance transformer-like circuit is needed to over-
come this low Q. With this circuit, a much higher Q is
achievable using high-Q external capacitors. This con-
figuration allows a tuning range of approximately
314MHz to 319MHz with 160kHz steps when resonated
with a 27nH inductor. Figure 2 shows a Q enhancement
circuit.
Startup Sequence
The MAX1474 must be programmed before use. When
powered up, the following sequence must occur:
• Deassert both EN and DAT.
• Assert EN.
• Send pulses on DAT. The total number of pulses
determines the capacitance value (Table 1).
• Deassert EN.
Applications Information
The MAX1474 offers users a wide range of applica-
tions, including trimming of regenerative receivers,
oscillators, RFID tags, and capacitive sensors. A
capacitive sensor is usually made of two capacitors.
One capacitor (CM) changes with the sensed function,
such as pressure, acceleration, or humidity. The sec-
ond capacitor (CR) is used as a reference capacitance
that does not vary with the sensed function. In most
applications, it is needed to detect the difference
between these capacitance values. Due to manufactur-
ing tolerances, there are normally offsets between CM
and CR, which can easily be nulled out by the FLECAP
as shown in Figure 4.
To increase the adjustment capacitance range, several
FLECAPs can be placed in parallel. Several capacitors
can be tied together to increase the number of steps or
the resolution, as shown in Figures 5a and 5b.
The FLECAP zero step differential capacitance is low,
typically 0.42pF. The parasitic capacitances are the
reason for the 6pF of input capacitance and the
decrease in range when configured as a single-ended
capacitor. The MAX1474 is an excellent choice in appli-
cations where only differential capacitance matters.
Table 1. Capacitance Values
NUMBER OF
PULSES ON
DAT
TYPICAL
VALUES
(CM = GND)
(pF)
TYPICAL
VALUES
( D I F F ER EN T IA L )
(pF)
0
6.41
0.424
1
6.63
0.769
2
6.86
1.11
3
7.09
1.45
4
7.31
1.79
5
7.54
2.13
6
7.78
2.47
7
8.00
2.81
8
8.22
3.13
9
8.45
3.48
10
8.67
3.82
11
8.90
4.16
12
9.12
4.49
13
9.34
4.84
14
9.57
5.18
15
9.77
5.52
16
10.02
5.84
17
10.25
6.18
18
10.47
6.53
19
10.67
6.87
20
10.87
7.20
21
11.10
7.54
22
11.33
7.88
23
11.57
8.22
24
11.80
8.55
25
12.02
8.89
26
12.24
9.24
27
12.46
9.58
28
12.68
9.91
29
12.91
10.25
30
13.14
10.59
31
13.33
10.93
_______________________________________________________________________________________ 5