Filtering Vout
The output Vout has a very low output impedance of
typically 2 Ω; it can drive 100 pF directly and shows
50% overshoot with approximately 1 nF capacitance.
Adding Rf allows much larger capacitive loads. Note
that with Rf of only 20 Ω, the load capacitor should be
either smaller than 1 nF or larger than 33 nF to avoid
overshoot; with Rf of 50 Ω this transient area is avoided.
When the Ref pin is connected to a load, due to the
leakage current and internal resistance (680 Ω),
VREF OUT (internal reference) can change and reduce
the measuring range. To guarantee the measuring
range:
• The leakage current from the Ref pin (source)
must be lower than 350 µA when the load is
connected to a voltage > 2.5 V.
Empirical evaluation may be necessary to obtain
optimum results.
• The leakage current from the Ref pin (sink) must
be lower than 4.4 µA when the load is connected
to a voltage < 2.5 V.
Example: Filtering the typical 450 kHz frequency of the
detector:
To have an attenuation of 20 dB at 450 kHz, the cutting
frequency of the 1st order filter is chosen at Fc=45 kHz.
• In the Ref in mode, an external reference voltage is
connected to the Ref pin; this voltage is specified
in the range 0 to 4 V and is directly used by the
transducer as the reference point for measurements.
To avoid transient area, the resistance Rf is chosen at
50 Ω.
The external reference voltage VREF must be able:
The filter capacitor is then as per Fig. 11.
Vref − 2.5
Load output resistance: RL
The minimum load resistance of Vout is 1 kOhm.
• Either to source a typical current of
maximum value will be 2.2 mA typ. when VREF = 4 V.
, the
680
2.5 − Vref
• Or to sink a typical current of
, the
680
Reference Voltage
maximum value will be 3.68 mA typ. when VREF = 0 V.
If the Ref pin of the transducer is not used it must be
left unconnected.
By using an external reference, it is easier to connect
the transducer to devices such as an ADC.
No special filtering is needed for Ref pin.
The Ref pin has two modes Ref in and Ref out:
In most applications, the output of the transducer is
connected to an ADC whose output is processed by
a DSP or a microcontroller.
• In the Ref out mode, the 2.5 V internal precision
reference is used by the transducer as the reference
point for bipolar measurements; this internal reference
is connected to the Ref pin of the transducer through
a 680 Ω resistor. It tolerates sink or source currents
up to 5 mA, but the 680 Ω resistor prevents this
current to exceed these limits.
The internal reference of these DSPs or ADCs can go
down to 1.8 V.
In this application, if you have an internal reference
in the DSP with external access, you can supply the
transducer’s reference in with it.
Fig. 11.
1
1
Cf =
Cf =
2 ∗π ∗ Rf ∗ Fc
2 ∗π ∗50∗ 45E 3
Cf ≥ 70.7nF
Ω
Rf = 50 _
Cf = 82 nF
0V
12
LEM [ LEM ]
