13

LT1394

Fast, High Impedance, Variable Threshold Trigger

A frequent requirement in instrumentation is a fast trigger

with a variable threshold. Often, a high impedance input is

also required. Figure 15 meets these requirements. Com-

parator C1 is the basic trigger, with threshold voltage set at

its negative input. Source follower Q1 provides high

impedance with about 2pF input capacitance and 50pA bias

current. Normally, Q1’s source bias point would be uncer-

tain and drifty, but stabilization techniques eliminate this

concern. A1 measures filtered versions of Q1’s gate and

source voltages. A1’s output biases Q2, forcing Q1’s

channel current to whatever value is required to equalize

A1’s inputs, and hence Q1’s gate and source voltages. A1’s

input filtering and roll-off are far slower than input frequen-

cies of interest; its action does not interfere with the

circuit’s main signal path. The 330pF capacitor prevents

fast edges coupled through Q2’s collector base junction

from influencing A1’s operation.

Q1 should contribute negligible timing error to minimize

overall delay. Figure 16’s photo verifies Q1’s wideband

operation. Trace B, Q1’s source, lags the input (Trace A) by

only 300ps. Input, FET buffer output and C1 output appear

as Traces A, B and C, respectively in Figure 17. As before,

the FET buffer is seen to contribute small timing error, and

C1’s output is about 8ns delayed from the input.

–

+

C1

LT1394

–

+

A1

LT1097

10M

5V

±3V

0.01

µF

0.1

µF

OUTPUT

1394 F15

10k

Q2

2N3904

INPUT

±3V

Q1

2N5486

1.5k

–5V

0.1

µF

10M

100

Ω

330pF

Figure 15. Buffer Provides 2pF, 50pA Input Characteristics for

Fast Trigger. Amplifier-Stabilized Biasing Eliminates FET Offset

200ps/DIV

1394 F16

Figure 16. Trigger Buffer’s 300ps Delay Minimizes

Timing Error. 4GHz Sampling Oscilloscope’s Output Is

a Series of Dots

A = 1V/DIV

B = 1V/DIV

Figure 17. Input (Trace A), FET Source (Trace B)

and Output (Trace C) Waveforms for the Trigger.

Total Delay Is 8ns

10ns/DIV

1394 F17

A = 1V/DIV

B = 1V/DIV

C = 2V/DIV