VFC320

4

SBVS017A

DISCUSSION OF

SPECIFICATIONS

LINEARITY

Linearity is the maximum deviation of the actual transfer

function from a straight line drawn between the end points

(100% full scale input or frequency and 0.1% of full scale

called zero.) Linearity is the most demanding measure of

voltage-to-frequency converter performance, and is a func-

tion of the full scale frequency. Refer to Figure 1 to deter-

mine typical linearity error for your application. Once the

full scale frequency is chosen, the linearity is a function of

operating frequency as it varies between zero and full scale.

Examples for 10kHz full scale are shown in Figure 2. Best

linearity is achieved at lower gains (

∆f

tion as close to the chosen full scale frequency as possible

The high linearity of the VFC320 makes the device an

excellent choice for use as the front end of Analog-to-Digital

(A/D) converters with 12- to 14-bit resolution, and for

highly accurate transfer of analog data over long lines in

noisy environments (2-wire digital transmission.)

Figure

Figure

Figure 1. Linearity Error vs Full Scale Frequency.

Figure 2. Linearity Error vs Operating Frequency.

0

2k

4k

6k

8k

10k

Operating Frequency (Hz)

0.003

0.002

0.001

0

–0.001

–0.002

–0.003

1k

3k

5k

7k

9k

B Grade

C Grade

f

Typical, T

FREQUENCY STABILITY VS TEMPERATURE

The full scale frequency drift of the VFC320 versus tem-

perature is expressed as parts per million of full scale range

per

°C. As shown in Figure 3, the drift increases above

10kHz. To determine the total accuracy drift over tempera-

ture, the drift coefficients of external components (espe-

RESPONSE

Response of the VFC320 to changes in input signal level is

specified for a full scale step, and is 50ns plus 1 pulse of the

new frequency. For a 10V input signal step with the VFC320

operating at 100kHz full scale, the settling time to within

±0.01% of full scale is 10µs.

THEORY OF OPERATION

The VFC320 monolithic voltage-to-frequency converter pro-

vides a digital pulse train output whose repetition rate is

directly proportional to the analog input voltage. The circuit

shown in Figure 4 is composed of an input amplifier, two

comparators and a flip-flop (forming a on-shot), two switched

current sinks, and an open collector output transistor stage.

Essentially the input amplifier acts as an integrator that

produces a two-part ramp. The first part is a function of the

input voltage, and the second part is dependent on the input

voltage and current sink. When a positive input voltage is

stant current sink is disabled by the switch. Note, this period

When the ramp reaches a voltage close to zero, comparator

A sets the flip-flop. This closes the current sink switches as

–7.5V. Note this ramp period is dependent on the 1mA

current sink, connected to the negative input of the op amp,

as well as the input voltage. At this –7.5V threshold point

Figure 3. Full Scale Drift vs Full Scale Frequency.

1k

1M

Full Scale Frequency (Hz)

1000

10

10k

100k

100

C Grade

B and S Grades

1k

1M

Full Scale Frequency (Hz)

0.10

0.001

10k

100k

0.01

D

T